NetApp volume/snapshot delete performance fix

NetApp volume/snapshot delete performance fix

cDOT NFS NetApp drivers back cinder volumes with array-side files and
use array-side cloning techniques when creating snapshots and when
creating volumes from glance images.  Customers have experienced
performance issues on the array when deleting many of these in
in quick succession, especially for large volumes.

This commit addresses these performance issues by overriding parent
class volume and snapshot delete methods, which use OS "rm" command to
delete volume backing files, with methods that use DOT API to delete
these files, and which invoke the optimized file deletion engine
available in DOT 8.3.

Closes-bug: 1497258
Change-Id: I44428d0840f6584f93ca214a2a607869b345554c