ceph: adjust crush tunables via 'ceph osd crush tunables <profile>'

Make it easy to adjust crush tunables.  Create profiles:

 legacy: the legacy values
 argonaut: the argonaut defaults, and what is supported.. legacy! (*(
 bobtail: best that bobtail supports
 optimal: the current optimal values
 default: the current default values

* In actuality, argonaut supports some of the tunables, but it doesn't
  say so via the feature bits.

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