Add whitelist option for yum repository files

[darxriggs]

This implements #485.

[rndmh3ro]

Hey @darxriggs,

thanks for your PR!

Two things:

• Can you please rebase onto master?
• I'd like to have some tests for this. We already have some tests for pam and I'd like to have some yum-repo tests in the same format: https://github.com/dev-sec/ansible-collection-hardening/blob/master/molecule/os_hardening/verify.yml#L40. Meaning a separate file with tests that gets included in the verify.yml. The tests should probably check if a repo from the whitelist still exists if its defined in the whitelist-variable.

[darxriggs]

Rebase done.

[darxriggs]

I will have a look at the tests in the next days.

[darxriggs]

I have added a test.

[rndmh3ro]

Lgtm!

I'd have preferred to user assert tasks to test the repository files, but this is okay, too.