Auto generate an FSID

It has becomes really anoying to manually generate an fsid prior to the
inital bootstrap. This commit introduces a method that auto-generates an
fsid. If for whatever reasons you want to force your own fsid you can
simply edit these 3 files and override the fsid variable:

- roles/ceph-common/vars/main.yml
- roles/ceph-mon/vars/main.yml
- roles/ceph-osd/vars/main.yml

Signed-off-by: Sébastien Han <sebastien.han@enovance.com>