osd/OSDMonitor: accept 'osd pool set ...' value as string

Newer monitors take this as a CephString.  Accept that so that if we are
mid-upgrade and get a forwarded message using the alternate schema from
a future mon we will handle it properly.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>