restapi : Add /clusters sub-collection under /datacenters/xxx(#674521)

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

Change-Id: I6b27ef501324d890efbe0e8b914d9db47653f146
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=674521
Signed-off-by: Ravi Nori <rnori@redhat.com>