webadmin: Moved NetworkModel command initiation to constructor

It had been previously performed only following some backend queries,
but I couldn't find a reason why; the command initialization doesn't
seem to depend on the state of NetworkModel.

Change-Id: I56f3a54081b00e8f8fb0a47f9d2d4234cdb6bc24
Signed-off-by: Lior Vernia <lvernia@redhat.com>