Largely inspired and taken from Kranthi's PS.
- Add support for creating custom CRUSH rules based off of failure
domains and device classes (ssd & hdd)
- Basic logic around the PG calculator to autodetect the number of
OSDs globally and per device class (required when using custom crush
rules that specify device classes).
Change-Id: I13a6f5eb21494746c2b77e340e8d0dcb0d81a591
6e8c289c13