PureISCSIDriver:Handle delete called on already deleted volume

If the delete_volume method gets an error from Purity
saying volume does not exist, don't proceed with attempt
at deletion

Change-Id: I2a830e43c876433da4cdb50eabe4fb66b7eb6faf
Closes-Bug: #1395826
Merge "PureISCSIDriver:Handle delete called on already deleted volume"