Add CFME_TESTS_KEY_FILE support

I also changed the conf loading logic to simplify it: If both an eyaml and yaml
exist in conf/, the unencrypted yaml is loaded and a message is logged saying so.
Writing out the unencrypted yaml defeats the encryption for a use-case you may not have been made aware of, which is the same one that `CFME_TESTS_KEY_FILE` addresses: Someone with read access to the jenkins UI.