VxFlexOS is rebranded to Powerflex.
cinder::backend::dellemc_vxflexos is deprecated,
use rebranded resource cinder::backend::dellemc_powerflex instead.
Depends-On: https://review.opendev.org/#/c/742016/
Change-Id: Ib88500a765c29a887c58c21d31cd39fcc43c7f6c