webadmin: Add warning to Setup networks dialog

Add warning to Setup networks dialog and use it to warn the user
on moving display network.

Change-Id: I35f0e019b6f77ab90fec89f614b49db589602353
Bug-Url: https://bugzilla.redhat.com/1078836
Signed-off-by: Yevgeny Zaspitsky <yzaspits@redhat.com>