When creating a network the management network should be chosen

- If the management network is in the valid network list, it should be the
default selection.
- If empty network is part of the valid network list it should be the last
one displayed.

Change-Id: I196b5381bacf6352397eaecf625333ece93de785
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=903287
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>