Add tidy linting checks for buildbot_steps.yml

This commit adds tidy checks for buildbot_steps.yml, as well as unit
tests. These checks include:

* Checking buildbot_steps.yml can be parsed by a YAML loader
* buildbot_steps.yml does not contain duplicate keys
* buildbot_steps.yml keys map to a list of strings

python/requirements.txt

@@ -12,6 +12,9 @@ flake8 == 2.4.1
 pep8 == 1.5.7
 pyflakes == 0.8.1
 
+# For buildbot checking
+PyYAML == 3.12
+
 # For test-webidl
 ply == 3.8