drop --cluster from ceph-disk activate (osd).

This commit removes the --cluster option from the 'ceph-disk activate'
part of the osd configuration (when the cluster is given as an arg).
Only 'ceph-disk prepare' accepts a cluster option.

Change-Id: I4027690c36141a184c1fb1b64b382e17cbbcf487
Closes-Bug: #1323120
Merge "drop --cluster from ceph-disk activate (osd)."