Add validation of VolumeMounts

In general, I prefer ``for _, obj := range someSlice``. However, specific to this one (and another one above): Is there any reason we want to use ``[]api.ContainerManifest`` instead of ``[]*api.ContainerManifest``? I know we may want to have some kind of immutable semantics, but ``api.ContainerManifest`` may be very large and might be inefficient to copy it around no?