webadmin: New Network dialog checks system tree DC

The dialog now looks at the selected item in the system tree, and if it
has a data center ancestor (i.e. it's not the system node), then the DC
list box in the new network dialog is grayed out and populated
accordingly.

Change-Id: Iae792afee7a8ada1b91c9db2bb0a0166cb2e2ad5
Bug-Url: https://bugzilla.redhat.com/953054
Signed-off-by: Lior Vernia <lvernia@redhat.com>