engine: remove disabled external scheduler plugin

Added a new command to remove disabled external policy units
Added a new dialog to show all policy units, in this dialog, disabled
external policy units can be removed.

Change-Id: Ia03da19368b75b125aa3ea144b36016feddcb08c
Bug-Url: https://bugzilla.redhat.com/1014700
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>