Adds pool aware scheduling for HNAS drivers

Adds pool aware scheduling for HNAS drivers

This commit adds the pool aware feature for HNAS drivers. Now, each service
label in HNAS XML config will be considered as a pool unit. In order to be able
to cast a volume to a service, the admin must create 'volume_type's and each
volume type need to have the 'service_label' with the same tag set in the XML
config, and then the user can create a volume with the desired volume_type.

Implements:  blueprint hds-hnas-pool-aware-sched
Change-Id:  Iddff7ad606b06454458fc53bfc7b812f9815f0e1