webadmin: Warning when editing Network Provider URL

User is warned in case of changing Provider URL when it had already
provided networks. Also added infrastructure for easy addition in the
future of similar checks for other Provider types.

Change-Id: Id71b5f916258f0250c2d5f1c6d77e9c1a9a9516e
Signed-off-by: Lior Vernia <lvernia@redhat.com>