Add default environment variable for Pipeline UUID

Add additional Pipelines default variables. This additional commit context would help with custom notifications, logging, etc. and could be used to determine pipeline workflow path (e.g. is this check-in from the automated merge or from a developer).

BITBUCKET_COMMIT_AUTHOR - Author display name responsible for current commit
BITBUCKET_COMMIT_MESSAGE - check-in message associated with current commit.
BITBUCKET_PIPELINE_ID - UUID of current pipeline (same id used in addon/pipelines/home#!/results/ link)