engine: Skip empty bridge mappings

If the user hasn't provided bridge mappings, there
is no need to pass it to ovirt host deploy.

Change-Id: Iaf2337d837ddb456f1c6bb70e2aec96394d2a2ea
Bug-Url: https://bugzilla.redhat.com/1093994
Signed-off-by: Moti Asayag <masayag@redhat.com>