mon/OSDMonitor: make 'osd pool mksnap ...' not expose uncommitted state

[This is a backport of d1501938f5d07c067d908501fc5cfe3c857d7281]

We were returning success without waiting if the pending pool state had
the snap.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>