Fix FSID not being set in ceph.conf

Symptom was that 'ceph --admin-daemon... config get fsid'
returned zeros, while correct fsid was present in cluster maps.
Fix it by populating FSID in ceph.conf, after extracting it from
monmap.
Merge pull request #162 from jcsp/fsid-conf

Fix FSID not being set in ceph.conf