engine: NetworkQoS validation in Add/Update Network

Added QoS-related validation in AddNetworkCommand and
UpdateNetworkCommand, to make sure that the QoS entity attached to the
network exists in the network's DC.

Change-Id: I70ecf50f5f786544cc4a68286ef9850c2b525026
Signed-off-by: Lior Vernia <lvernia@redhat.com>