ZFSSA iSCSI vol create fails with vol type option

Vol create with volume-type option is not working since
volume_backend_name contains the class name as
predefined string. No matter what was specified in cinder.conf
as volume_backend_name, volume creation failed.
Multi-backend option and using extra specs to create custom volumes
won't work.
The fix is to look whether volume_backend_name is part of the
configuration or falls into the class name in case there is
no backend name.

Closes-Bug: 1373621
DocImpact

(cherry picked from commit 5c61d57d3693523e9cbf11bf0b5b09bafe699247)

Change-Id: I1bc501dd4c5689d96c7beb720b64112df1770232