feature finish: Insert PT link into RB description

The Review Board description is no longer generated by rbt, it is
generated manually and passed to rbt using --description. This generated
description includes the link to the relevant Pivotal Tracker story on
the first line so that people can navigate there easily.

Change-Id: I22d1ec45595490bf3113cd7ae7ced13cc497a762
Signed-off-by: Ondrej Kupka <ondra.cap@gmail.com>