Per datastore volume support

Implements blueprint: per-datastore-volume-support

Introduces volume support on a datastore basis using
config values in datastore config groups.

DocImpact: New config values device_path and volume_support
for each datastore have been added, instead of the DEFAULT
conf section.

Change-Id: I871cbed1f825d719b189f71a3ff2c748fb8abdc0
Merge "Per datastore volume support"