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
Merge "VMWare - Check for compute node before triggering destroy"