VMWare - Check for compute node before triggering destroy

While booting an instance, if Claim on that compute node is not successful
compute manager triggers a call to destroy instance. Destroy fails since the
compute node (Cluster/ESX host) is set only after claim is successful.
Hence the check for node is made before destroy, to ensure that exception
does not get thrown. This ensures that the instance gets rescheduled
Closes-Bug: #1307408

Change-Id: Iaaf931e9c1e6cf046497e2c64952f92e802ad4be
(cherry picked from commit aadfffaaa69c6a8939f6c88cfbaeb026c7604163)
Merge "VMWare - Check for compute node before triggering destroy" into stable/icehouse