RFR: Support yaml metadata for actions and rules

* Add: Yaml support for actions and rules meta. Both yaml and yml extensions
are supported.
* Add: yaml, yml and json meta for sample action in fixtures for testing.
Oops. Good catch. Fixed https://github.com/StackStorm/st2/commit/0b29ee23b9f98da0b24e2444245cddd7fd1bd270