Allow for eradicating Pure volumes on Cinder delete

Allow for eradicating Pure volumes on Cinder delete

This change adds in a config option which, when enabled, will cause
the Pure Storage volume drivers to eradicate volumes, snapshots, and
protection groups on delete for their Cinder counterpart.

With this config enabled an admin will NOT be able to recover data once
objects are deleted. By default the new config option is disabled and
volumes, snapshots, and protection group snapshots will be retained
pending eradication where they can be recovered for a period of time.

DocImpact
Closes-Bug: #1537187

Change-Id: Ib3656bb64e5104f6bd05d28535cf91a46a038b55