engine: Do not invoke CommitNetworkChanges if not needed

There is no reason to sent persist network changes if
no changes were detected via setupNetworks.

Change-Id: Ie06d43ffaf899af4749bb1221764d40fab10a572
Bug-Url: https://bugzilla.redhat.com/1061724
Signed-off-by: Moti Asayag <masayag@redhat.com>