gluster-swift-gen-builders behaviour changes

gluster-swift-gen-builders copies the older ring files to
/etc/swift/backup_ring. And then creates the new ring files to /etc/swift

BUG: 986458

Change-Id: I06e93179e46da56d5046419e66c8864aa08f184d
Signed-off-by: Chetan Risbud <crisbud@redhat.com>
Reviewed-on: http://review.gluster.org/5384
Reviewed-by: Luis Pabon <lpabon@redhat.com>
Tested-by: Luis Pabon <lpabon@redhat.com>
Reviewed-on: http://review.gluster.org/5457