Add UCM ClearCase activity support using parse_revision_spec

This change adds the ability to post change associated with UCM activities in
clearcase. The command to do so is `rbt post activity:<activity_name>[@pvob]`

This was initially posted at https://reviews.reviewboard.org/r/5168, prior to
the parse_revision_spec framework.

Testing Done:
Can post a review request from RHEL6 (python 2.6) using rbt post
activity:<activity_name>

Bugs closed: 3179

Reviewed at https://reviews.reviewboard.org/r/6235/