Implement NetApp SolidFire Multipath

The SolidFire driver only provides to OpenStack Nova the primary SVIP (default, created during cluster setup), or a suser defined SVIP through the cinder.conf parameter sf_svip [1]. This means that even if we create new SVIPs through the use of the VLAN feature in SF, Nova and the os-brick will never get this information, opening only one iSCSI connection to each volume, despite the existence of multiple data paths available.

This implementation will allow the use of multiple SVIP in SolidFire, in order to enable multipath support. The Cinder configuration attribute "sf_svip" should accept a list of SVIPs, which will be checked against the available VLANs list or primary SVIP.

[1] https://netapp-openstack-dev.github.io/openstack-docs/train/cinder/configuration/cinder_config_files/section_solidfire-conf.html