mgr/snap_schedule: validate fs before execution

mgr/snap_schedule: validate fs before execution

Stop command execution if there are more than one filesystem and --fs
argument is missing in command-line, i.e. do not use the first fs in the
fsmap if there are more than one filesystem.
This is to ensure that user doesn't mistakenly run the command against
the first fs by missing to specify the desired fs.

Fixes: https://tracker.ceph.com/issues/62218
Signed-off-by: Milind Changire <mchangir@redhat.com>