Only test for azure-rm and google features when enabled

[ekohl]

Why are you introducing these changes? (Problem description, related links)

While developing #511 I wanted to deploy a plain Foreman without any plugin. I noticed these tests failed. These features are optional and they should only run when the features are present.

What are the changes introduced in this pull request?

• The compute resource tests are split into dedicated functions
• Where optional, the providers are properly marked by their feature.

How to test this pull request

Steps to reproduce:

• Deploy without azure-rm and google; verify all the test suite passes
• Deploy with azure-rm and google; verify all the test suite passes and it runs all tests

Checklist

• Tests added/updated (if applicable)
• Documentation updated (if applicable)