Release triggered statuses should be created on tags, not branch

Release triggered commit statuses will be attached to tags instead or release target, which was sometimes master branch.

Branch restrictions for `labeled` event triggers

Pipelines can now register to labeled event only on specific branch - enabling triggers to fire off only on specific PRs based on labels.

Bug and security fixes

• Fix issue where 504 from Jenkins causes error status on PRs (happens mostly during scheduling on Jenkins under high load)
• Fix issue where triggear tries to send status description longer than 140 characters, which is GitHub's limit causing 422 response
• Fix security issue with pyyaml dependency

Use MONGO_URL env var to connect to mongo DB

2.1.9

Always rely on MONGO_URL when connecting to mongodb