Revert "raise an error when trying to delete a terminated instance"

This reverts commit 0a95549507beae4f4db72696b88eb4f292aad8c7.
raise VMNotFound from AWS CPI #delete_vm when VM cannot be found

Signed-off-by: Dmitriy Kalinin <dmitriy@pivotallabs.com>
added cpi spec to make sure that aws, vsphere and openstack cpis raise VMNotFound error when deleting non-existent vm

Signed-off-by: Dmitriy Kalinin <dmitriy@pivotallabs.com>