Update engine's mgmt net to have the correct vlan id

Up until now, when setting the management network over a vlan, the
logical network in the engine would not be automatically updated to
match the real configuration. This addresses the issue.

Change-Id: Ica4c5804395d364438f1e142387376cc7adc7172
Bug-Url: https://bugzilla.redhat.com/1124207
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
(cherry picked from commit a6bd8963d3d82af57224227e5f2af7aef6cd3492)