Allow setting multiple availability zones per cinder-volume service

The direct motivation to start this work was change in Nova's behavior introduced by https://review.openstack.org/#/c/157041/. Nova started to pass AZ selected by user in create volume requests to Cinder. This turned out to be a problem for many operators as their compute AZs weren't matching storage AZs. In particular setups running Ceph were affected as storage resiliency can be addressed on Ceph level using CRUSH maps (http://ceph.com/docs/master/rados/operations/crush-map/). Such configuration doesn't care about storage AZ at all.

Naturally one may think that disabling AvailabilityZoneFilter makes Cinder to ignore AZ of create volume requests. Unfortunately validation of AZ name is done also on cinder-api level to inform user early of invalid AZ input. To address this issue in Liberty a https://review.openstack.org/#/c/217857/ is going to be merged in Liberty RC1, which makes Cinder to de-facto ignore AZ of incoming create volume requests if a certain option is set.

This blueprint aims at adding ability to specify multiple AZs per cinder-volume service.