Warn in logs if atlantis.yml is used instead of atlantis.yaml

[davidgereb]

Issue

Atlantis doesn't recognize the repo level config if the file is named atlantis.yml instead of atlantis.yaml.

Proposal

Warn in the logs if atlantis.yml is detected.
Make atlantis config files work with .yml file extension.

[lkysow]

I think it's simpler if it's always .yaml. But I'd support a warning in the logs if one with .yml is found.

[lkysow]

I'm going to change the title then to ask for a warning.

[mdcurran]

@lkysow are we good to close this? Also my b for forgetting to use the GH "fixes" syntax.

[lkysow]

Yes, I'll close it nice work!

[lkysow]

(Fixed by #816)