Missing a key for perm 'w' in permmap (src/pybin/ceph_rest_api.py:599)

Missing a key for perm 'w' in permmap (src/pybin/ceph_rest_api.py:599) ,which leads to a 500 error when getting mds help info via rest api.
We should either add a key for perm 'w' or change perm in src/mon/MonCommands.h

Fixes: #7180
Signed-off-by: Kai Zhang <kazhang2@cisco.com>