ceph-disk-activate: identify cluster .conf by fsid

Determine what cluster the disk belongs to by checking the fsid defined
in /etc/ceph/*.conf.  Previously we hard-coded 'ceph'.

Note that this has the nice side-effect that if we have a disk with a
bad/different fsid, we now fail to activate it.  Previously, we would
mount and start ceph-osd, but the daemon would fail to authenticate
because it was part of the wrong cluster.

Fixes: #3253
Signed-off-by: Sage Weil <sage@inktank.com>