Allow deletion of clone parents

[datamattsson]

CSI e2e tests fail due to the fact that the driver can't delete parents of cloned PVCs.

e2e test failure:

External Storage [Driver: csi.vastdata.com] [Testpattern: Dynamic PV (filesystem volmode)] multiVolume [Slow] should concurrently access the volume and its clone from pods on the same node [LinuxOnly] [Feature:VolumeSourceXFS]

This is a very misleading failure message but after an analysis of the test, the parent is deleted first and fails. VolumeSourceXFS is also very misleading but that was the name of the initial test for mounting cloned XFS volumes on the same host which had issues due to duplicate UUIDs. The test case name has remained the same but applies to any file-based CSI driver that have cloning capabilities.

The same e2e driver spec, StorageClass and VolumeSnapshotClass was used as in #19