modules/reviewboard: Check rbt version on init

Review Board module now ensures that the RBTools installed are of the
right version.

Implemented support for hooks into repo init. Any part of the code base
regiter a callback that will be run during repo init (app.Init) to check
some requirements. The hooks are registered via repo.AddInitHook

Change-Id: 3bdf7e2197
Story-Id: SF-11