Drivers were removed due to lack of CI.
Refactored iSCSI to use single target/tg with multiple zvols.
Migrated unit tests to use mock.
Added releasenotes.
DocImpact
Implements: blueprint nexentastor-4-cinder-drivers
Change-Id: I2ff4c79573322a9c0cd473a9d413e1fcdbe55ee0