mon/OSDMonitor: handle 'osd tier add ...' race/corner case

If you have two racing requests to add two different pools as a tier, the
committed checks will pass but they proposals will conflict.  Recheck the
pending pools for the same conditions and wait for a commit if they
occur.

Reported-by: Loic Dachary <loic@dachary.org>
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>
mon/OSDMonitor: fix some whitespace

Signed-off-by: Sage Weil <sage@inktank.com>
mon/OSDMonitor: fix race in 'osd tier remove ...'

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