improve "koji" parameter handling

[ktdreyer]

Our common_koji.get_profile_name() method seems to duplicate Ansible's own env_fallback() method. Maybe we could use env_fallback and drop some code / get nicer error messages? Currently get_profile_name() raises ValueError if it can't find a profile. env_fallback() will raise AnsibleFallbackNotFound, so I guess that's a tiny bit easier to catch?

From https://docs.ansible.com/ansible/latest/dev_guide/developing_program_flow_modules.html#fallback , it would look something like this:

koji=dict(fallback=(env_fallback, ['KOJI_PROFILE']))

Need to play around with this and see if it's viable and a net gain.

(Also, beware we have no integration tests for un-setting KOJI_PROFILE)