mon: Don't block on EAGAIN from `osd pool set`


This is a bit subjective, but thought it would be clearer to send a PR for discussion than to go straight to the mailing list. 

---

Behavior was changed in 69321bf5 to block instead of returning
when pg_num/pgp_num changes were prevented by PG creates in
progress.  That can be a long block, and there's no feedback
to user about why their command is stuck -- let's handle the
retry in the caller of the command instead.

Signed-off-by: John Spray <john.spray@inktank.com>