unlock VLAN Manager radiobutton from Network tabs

Now nova-network works properly in VLAN Manager mode. But element on UI
for setting this mode currently locked. It will be unlocked by this patch.

Change-Id: Id98250de449f9d9e6e4e61bc544746067fce729d
blueprint: vcenter-vlan-manager
Merge "unlock VLAN Manager radiobutton from Network tabs"