webadmin: Stopped progress bar after initializing network list

The code didn't stop the progress bar, so when a DC was selected in
the system tree (rather than a network) the progress bar would
continue indefinitely when the dialog was opened.

Change-Id: I469e4d95e479c99f5315fbc89f369e254b16cfd9
Bug-Url: https://bugzilla.redhat.com/1001000
Signed-off-by: Lior Vernia <lvernia@redhat.com>