mon/OSDMonitor: add flag `--yes-i-really-mean-it` for setting pool size 1

mon/OSDMonitor: add flag `--yes-i-really-mean-it` for setting pool size 1

Adds option `mon_allow_pool_size_one` which will be disabled by default
to ensure pools are not configured without replicas.
If the user still wants to use pool size 1, they will have to change the
value of `mon_allow_pool_size_one` to true and then have to pass flag
`--yes-i-really-mean-it` to cli command:

Example:
`ceph osd pool test set size 1 --yes-i-really-mean-it`

Fixes: https://tracker.ceph.com/issues/44025
Signed-off-by: Deepika Upadhyay <dupadhya@redhat.com>