webadmin: Cannot add network to VM when dc have bridgeless mgmnt network

If the management network is brigeless it is not in the vm network list.
If there are no other networks in the list it gets to the last "null"
network and caused NPE.

Change-Id: I9f844fde815c00dff441192d9e977e4c23615ce3
Bug-Url: https://bugzilla.redhat.com/911115
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>