Configurable volume storage_protocol + vendor_name

Allow storage_protocol and vendor name of configured backend
to be set in tempest.conf, allowing volume tests against drivers
other than the default (iSCSI).

(cherry picked from commit 3fc3eaea4c6a74f9fa8de8268a8594722ce740a0)

Conflicts:
	etc/tempest.conf.sample
	tempest/config.py