Add change task properties

Expose the following new properties for use in task config files:

 ${_change_number}
 ${_change_id}
 ${_change_project}
 ${_change_branch}
 ${_change_status}
 ${_change_topic}

These properties are readily available on ChangeData's and thus easy and
cheap to expose. Also, updates the respective documentation and
functional tests.

Change-Id: Iee851388af479c9aaadf8a78b00d536f8882d241
7 files changed