mon/FSCommands: Fix 'fs new' command

mon/FSCommands: Fix 'fs new' command

After creating a filesystem using the 'fs new' command, the value
of the 'data' and 'metadata' key of the datapool and metadatapool's
application tag 'cephfs' should be the filesystem's name. This
didn't happen when the data or metadata pool's application metadata
'cephfs' was enabled before the pool was used in the 'fs new' command.
Fix this during the handling of the 'fs new' command by setting the
value of the key of the pool's application metadata 'cephfs' to the
filesystem's name even when the application metadata 'cephfs' is
already enabled or set.

Fixes: https://tracker.ceph.com/issues/43761
Signed-off-by: Ramana Raja <rraja@redhat.com>