mon/OSDMonitor: take 'osd pool set ...' value as a string again

We ran into problems before when we made this a string because a mixed cluster
of mons might forward a client request with the wrong schema. To make this
work, we make the new code understand both the new and old schema, and also
backport a change to emperor and dumpling to handle the new schema.

For the previous attempt to do this, see:
337195f04653eed8e8f153a5b074f3bd48408998
2fe0d0d97af95c22db80800f5b9da51f672d9407

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