Fix OS-142: Lock source instance during bless

Source instance will no longer be able to be deleted, rebooted, etc. while being blessed.

Upon deletion "Cannot delete while blessing. Please try again later." is displayed.
Upon reboot, pause, etc. "ERROR: Cannot 'pause' while instance is in task_state blessing (HTTP 409)"
is displayed. This fix applies for both admin and regular users.

Change-Id: I8a849fbdacd56eee1b97925b65d99440a337c2ed