webadmin: Redesigned Add/Edit Network

Split the Add/Edit Network popup to General and Cluster tabs, and
accommodated attaching networks to a provider, in which case VLAN, MTU
and VM definitions cannot be changed (VM is true by default), and a
provider label may be provided.

Change-Id: I25b5533a41358d51b959f94f36e390f56f630d93
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>