Agent host terraform constraints (#387)

* Fix some formatting issues in the README.md

* Add variables to build constraints to the agent-host terraform module

agent/terraform/README.md

@@ -9,62 +9,71 @@ This terraform module assume that the Juju model you want to deploy
 to has already been created.
 
 1. First, create the model
-```
+
+    ```
     $ juju add-model agent-host-1
-```
+    ```
 
 2. Create ssh keys to use on the agent host
-```
+
+    ```
     $ ssh-keygen -t rsa -f mykey
-```
+    ```
 
 3. Create a git repo with the Testflinger configs
 
-If you have more than one agent host to deploy, create a separate directory
-for each of them in this repo. Then create a separate directory for each
-agent in the agent host directory where they reside. For example:
-
-```
-/agent-host-1
--/agent-101
-  -testflinger-agent.yaml
-  -default.conf
--/agent-102
-  -testflinger-agent.yaml
-  -default.conf
-/agent-host-2
-...
-```
+    If you have more than one agent host to deploy, create a separate directory
+    for each of them in this repo. Then create a separate directory for each
+    agent in the agent host directory where they reside. For example:
+
+    ```
+    /agent-host-1
+    -/agent-101
+      -testflinger-agent.yaml
+      -default.conf
+    -/agent-102
+      -testflinger-agent.yaml
+      -default.conf
+    /agent-host-2
+    ...
+    ```
 
 4. Create a main.tf which specifies the required parameters for this module
 
-```
-terraform {
-  required_providers {
-    juju = {
-      version = "~> 0.13.0"
-      source  = "juju/juju"
+    ```
+    terraform {
+      required_providers {
+        juju = {
+          version = "~> 0.13.0"
+          source  = "juju/juju"
+        }
+      }
+    }
+
+    provider "juju" {}
+
+    module "lab1" {
+      source = "/path/to/this/module"
+      agent_host_name = "agent-host-1"
+      juju_model = "agent-host-1"
+      config_repo = "https://github.com/path_to/config_repo.git"
+      config_branch = "main"
+      config_dir = "agent-host-1"
+      ssh_public_key = filebase64("mykey.pub")
+      ssh_private_key = filebase64("mykey")
     }
-  }
-}
-
-provider "juju" {}
-
-module "lab1" {
-  source = "/path/to/this/module"
-  agent_host_name = "agent-host-1"
-  juju_model = "agent-host-1"
-  config_repo = "https://github.com/path_to/config_repo.git"
-  config_branch = "main"
-  config_dir = "agent-host-1"
-  ssh_public_key = filebase64("mykey.pub")
-  ssh_private_key = filebase64("mykey")
-}
-```
+    ```
+
+    There are additional parameters you can adjust here if you want:
+     - **agent_host_cores**: (default: 4) Number of cpu cores to use for the agent host
+     - **agent_host_mem**: (default: "32768M") Amount of RAM to use for the agent host. This needs to be specified as a string with "M" at the end.
+     - **agent_host_storage: (default: 1048576M) Storage size for the agent host. This needs to be specified as a string with "M" at the end.
+     - **override_constraints**: By default, the constraints passed to Juju will use the previous parameters to build something in this format: `arch=amd64 cores=${var.agent_host_cores} mem=${var.agent_host_mem} root-disk=${var.agent_host_storage} root-disk-source=remote virt-type=virtual-machine`. If you need to override this entire line to send it something completely different, use this and the previous `agent_host_*` parameters will be ignored.
+
 
 5. Initialize terraform and apply
-```
+    ```
     $ terraform init
     $ terraform apply
-```
+    ```
 

agent/terraform/locals.tf

@@ -0,0 +1,3 @@
+locals {
+  agent_host_constraints = try(var.override_constraints, "arch=amd64 cores=${var.agent_host_cores} mem=${var.agent_host_mem} root-disk=${var.agent_host_storage} root-disk-source=remote virt-type=virtual-machine")
+}

agent/terraform/main.tf

@@ -1,6 +1,7 @@
 resource "juju_application" "testflinger-agent-host" {
-  name  = var.agent_host_name
-  model = var.juju_model
+  name        = var.agent_host_name
+  model       = var.juju_model
+  constraints = local.agent_host_constraints
 
   units = 1
 

agent/terraform/variables.tf

@@ -8,6 +8,30 @@ variable "agent_host_name" {
   description = "Name of the agent host juju application"
 }
 
+variable "agent_host_cores" {
+  type        = number
+  description = "Number of cpu cores to use for the agent host"
+  default     = 4
+}
+
+variable "agent_host_mem" {
+  type        = string
+  description = "Amount of RAM to use for the agent host"
+  default     = "32768M"
+}
+
+variable "agent_host_storage" {
+  type        = string
+  description = "Storage size for the agent host"
+  default     = "1048576M"
+}
+
+variable "override_constraints" {
+  type        = string
+  description = "Use if you need to override the constraints built with the other agent_host_* vars"
+  default     = ""
+}
+
 variable "config_repo" {
   type        = string
   description = "Repository URL for the agent configs on this agent host"
@@ -33,3 +57,4 @@ variable "ssh_private_key" {
   description = "base64 encoded ssh private key to use on the agent host"
 }
 
+