engine: Empty SetupNetworksCommand isn't sent to VDSM

Changed the SetupNetworksCommand so that if it doesn't change anything
in a host's network configuration, it does nothing (instead of sending
an empty command to VDSM) and returns success.

Change-Id: Ib68eeb1e7e75aea7b0730ee4650675fd2b9d37cd
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=904743
Signed-off-by: Lior Vernia <lvernia@redhat.com>