osd: make default bloom hit set fpp configurable

Signed-off-by: Sage Weil <sage@inktank.com>
mon/OSDMonitor: add 'osd tier add-cache <pool> <size>' command

This is a friendlier interface for setting up a cache tier with some
reasonable defaults (defined via config options).  This will simplify
the user experience and documentation.

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