review_board: Register hook iff module active
review_board: Register hook iff module active

Right now the repo init hook to check for RBTools is always run and it
doesn't matter what code review module is being used.

This patches changes this so that the repo init hook is only run when
the Review Board code review module is active.

Change-Id: af9d26bbf2
Story-Id: SF-162

salsaflow/salsaflow,salsaflow/salsaflow