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.
Save functionality was already there, but it's not a great process. This should work, but isn't tested:
```python
from utils import conf, _conf

# load from eyaml
assert conf.credentials
# save to yaml
conf.save('credentials')
# edit, then save
_conf.encrypt_yaml('credentials')
```