mon/OSDMonitor: add 'mon osd allow primary affinity' bool option

mon/OSDMonitor: add 'mon osd allow primary affinity' bool option

By default, disallow adjustment of primary affinity unless the user has
opted in by adjusting their monitor config.  This will avoid some user
pain because inadvertantly setting the affinity will prevent older clients
from connecting to and using the cluster.

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