webadmin: Attach imported networks to clusters

This patch causes the import operation to not only add the network to
the data center, but also attach it to all its clusters as a
non-required network.

Change-Id: I25291f71d0966019a7cfb52bb66d10f9cc9b51e6
Signed-off-by: Lior Vernia <lvernia@redhat.com>