Add an enable_cinder_backend_pure_iscsi and
enable_cinder_backend_pure_fc options to etc/kolla/globals.yml
to enable use of the FlashArray backend.
Update the documentation to include a section on configuring
Cinder with the FlashArray.

Implements: blueprint pure-cinder-driver
Change-Id: I464733f1322237321ed1ffff8636cf30bd1cbb38