ceph-crush-location: new crush location hook

This generalizes the bit of code that builds a key=value pair list to
update an entity's CRUSH location.

Signed-off-by: Sage Weil <sage@inktank.com>
upstart, sysvinit: use ceph-crush-location hook

Instead of hard-coding a check in ceph.conf and some reasonable
defaults, defer this work to ceph-crush-location, and allow users to
specify their own hook with alternative logic.

This can be helpful in a nubmer of cases, like:

 - rack (or other) information included in hostname and easily parsed
   out by a hook
 - multiple types of devices in each host, resulting in 'parallel'
   crush trees (e.g., one for hdd, one for ssd)

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