Add multiple NexentaStor hosts/volumes support in nexenta iSCSI volume driver.

There are many use-cases where we need to support multiple hosts to choose from to serve as a backend for Cinder volume or backup. This is easily supported in NFS driver via 'nfs_shares' config option in Cinder.
In Nexenta iSCSI driver we can add similar behavior by specifying another config file named, say, 'nexenta_volume_hosts' with the similar format. The file will contain lines looking as: 'nexenta_volume http[s]://user:pass@host:[port]'. Each line will be read during driver initialization and NexentaStor NMS service will be connected using passed params. Then we can choose between different backend hosts to perform volume creation/backup based on free capacity (as done in NFS driver) or other characteristics.
Please proceed to https://review.openstack.org/#/c/47005/ for code review.