VMware: delete vm snapshot after nova snapshot

When performing a nova snapshot, the vmware driver takes a VM snapshot
and copies out the snapshotted disk. After the operation, the VM
snapshot should be deleted, to prevent growing disk chain(s) on each nova
snapshot.

Closes-Bug: #1252967
Change-Id: I7861ccce0153e45eea0316a2e8dd824210db1660
Merge "VMware: delete vm snapshot after nova snapshot"