mon: warn if crush has non-optimal tunables

Allow warning to be disabled via ceph.conf.  Link to the docs from the
warning detail.  Add a section to the docs specifically about what to do
about the warning.

Signed-off-by: Sage Weil <sage@inktank.com>
crush: expand info about tunables that we dump

Include the tunables profile, and flags indicating whether it is optimal,
legacy, or requires certain features.

Signed-off-by: Sage Weil <sage@inktank.com>
mon/OSDMonitor: 'osd crush show-tunables'

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