restapi : Add /networks sub-collection under /datacenters/xxx(#741111)

Adds datacenters/networks links to see specific data center networks.

Change-Id: Iacc4a480e3a1270cfcab022549e7f53cfc352c6e
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=741111
Signed-off-by: Ravi Nori <rnori@redhat.com>