mon: fix 'mds set allow_new_snaps'

We had already added this as a flag (set/unset) when I generalized the
'mds set_max_mds' to be 'ceph mds set <var> <val>'.  Switch the snaps
flag to be a key/value to with true/false (similar to the hashpspool
pool flag) since there are fewer users and the var/val approach is more
general.

Signed-off-by: Sage Weil <sage@inktank.com>