Make fsid comparison case-insensitive

get_fsid and find_cluster_by_uuid are modified so ceph-disk activate and
ceph-disk activate-all will work if the fsid uses uppercase characters.

Signed-off-by: Harry Harrington <git-harry@live.co.uk>
Merge pull request #684 from git-harry/ceph_disk_lowercase_fsid

Make fsid comparison case-insensitive

Reviewed-by: Sage Weil <sage@inktank.com>