engine: Generalize messaging broker type

The patch starts a series of refactoring patches
to support various messaging broker types to be
configured on hosts which are installed with an
external network provider.

Change-Id: Ib8075f2ef1227ab9c4a40cd3a961921d42d5ac22
Signed-off-by: Moti Asayag <masayag@redhat.com>