webadmin: Validate no duplicate network names on import

It is reasonably easy to detect whether networks with duplicate names
are attempted to be imported, so this can be checked before the action
is sent to the backend.

Change-Id: If63d450bfc8d666ebb383e29fdfe43fcf879db78
Bug-Url: https://bugzilla.redhat.com/1048752
Signed-off-by: Lior Vernia <lvernia@redhat.com>