provider/vSphere: Make env destroy configurable

This patch introduces the environment variable
`VSPHERE_DESTROY_ENABLED`. Unless set to `true`, an environment deployed
to vSphere will not be destroyed during the deprovision step.

This PR is related to crosscloudci#161 and possibly crosscloudci#154.

provision.sh

@@ -436,7 +436,11 @@ elif [[ "$CLOUD_CMD" = "vsphere-deploy" || \
         terraform taint -module=kubeconfig null_resource.kubeconfig || true
         time terraform apply -auto-approve ${DIR}/vsphere
     elif [ "$CLOUD_CMD" = "vsphere-destroy" ] ; then
-        time terraform destroy -force ${DIR}/vsphere || true
+        if [ "${VSPHERE_DESTROY_ENABLED}" = "true" ]; then
+          time terraform destroy -force ${DIR}/vsphere || true
+        else
+          echo "vsphere environment destruction disabled"
+        fi
         # Exit after destroying resources as further commands cause hang
         exit
     fi