webadmin: change cluster-policy header text

Change cluster column header from "Attached Clusters"
 to "Clusters on this policy"

Change-Id: I1e25cfbeb2c21566fd22c5adb144fcbc225beec2
Bug-Url: https://bugzilla.redhat.com/995238
Signed-off-by: Noam Slomianko <nslomian@redhat.com>