Fix code scanning alert - Request without certificate validation

[flemk]

Tracking issue for:

• https://github.com/flemk/python-licensing/security/code-scanning/1

License check HTTPS request is made without certificate verification.
Since (at least in the Dockerfile) a self-signed certificate is generated, the request should be checked against it for security' sake:

response = requests.get(
    params = ...
    verify='/path/to/cert.pem',
    ... )

• Give the user freedom to use a custom (pre-made) certificate
  • Possibility to inject a custom certificate ->ENV or mounted volume to not use self-signed certificate
• Save the generated certificate in volume so it won't get lost
• For self-signed certificate verification it'd be necessary to ship the cert.pem to end-user in licensed package
  • Invent a fancy way to inject the cert.pem in package or remind the user in README / HOWTO to add the cert.pem to package