Update buildbot_steps lint to handle env variables

https://github.com/servo/saltfs/pull/687 added support
for specifying environment variables in `buildbot_steps.yml`.
Update the servo-tidy buildbot_steps.yml linter to reflect this.

Use the voluptuous Python library (BSD 3-clause license) for validation
in lieu of a much larger hand-written implementation.
Update the tidy self tests to take into account the new error messages.

python/requirements.txt

@@ -15,6 +15,7 @@ pep8 == 1.5.7
 pyflakes == 0.8.1
 
 # For buildbot checking
+voluptuous == 0.10.5
 PyYAML == 3.12
 
 # For test-webidl