osdmap: make replica separate in default crush map configurable

Add 'osd crush chooseleaf type' option to control what the default
CRUSH rule separates replicas across.  Default to 1 (host), and set it
to 0 in vstart.sh.  This captures the desired effect of 7ea5d84f but
avoids breaking vstart, and gives users of mkcephfs (or any other
cluster creation tool) flexibility.

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