mon/FSCommands: add 'recover' flag in `fs new` command

mon/FSCommands: add 'recover' flag in `fs new` command

Currently, to recover a file system after recovering monitor store, you
need to stop all the MDSs; create FSMap with defaults using `fs new`
command; execute `fs reset` command to get the file system's rank 0 into
existing but failed state; and then restart MDSs.

Add 'recover' flag to the `fs new` command that sets the file system's
rank 0 to existing but failed state, and sets the file system's
'joinable' setting to False. Using the `fs new` command with 'recover'
flag gets rid of the steps to stop all the MDSs and execute `fs reset`
command when recovering the file system after recoving monitor store.

Fixes: https://tracker.ceph.com/issues/51716
Signed-off-by: Ramana Raja <rraja@redhat.com>