Add support for default node fallback.

[notpeter]

Add an additional optional configuration parameter 'default_node' which when a node is lacks a yml config file it falls back to provide data from this default node. The point of this is to allow the equivalent of '*': in a salt pillar/top.sls file. With this patch you can still target pillar data to minions which you don't have corresponding minion_id nodes defined. Either by hard coding parameters/classes in the default_node yaml file or with class_mappings regexes in the reclass-config.yml.

This parameter defaults to None when unset. This causes the same behavior as pre-patch (exception thrown on node not found).

I haven't pushed this change into the salt top module because I don't use this functionality and so can't easily test. I believe this behavior could be implemented by adding a default_node kwarg to top() and when storage adaptor is initialized. I don't have ansible infrastructure to test with so I've made no changes to the ansible/* code. reclass-ansible behavior appears unchanged (as expected) since it's just ignoring an extra kwarg passed from the cli module.

[madduck]

The problem here is that now reclass cannot enumerate all nodes anymore. At the moment, Salt cannot use this functionality, since it actually doesn't track which nodes it contacts (Ouch! saltstack/salt#2361), but other tools supported by reclass certainly do.

There was another idea floating around, #9 and #25, i.e. node aliases. This could be extended to support enumeration globs, e.g. you create a file default_www.yml and in it, you define e.g.

applies_to:
    - www.{000..999}.*

How do you feel about that?