nas_secure_file_permissions not followed for NFS snapshots

[Marc Methot]

NFS snapshots copy forces permissions '666'.

When creating a snapshot the permissions are applied adequately, however this is not the case when creating an instance from snapshot.

This is due to the use of `_set_rw_permissions_for_all`

~~~
./cinder/volume/drivers/nfs.py
~~~
    def _copy_volume_from_snapshot(self, snapshot, volume, volume_size):
        """Copy data from snapshot to destination volume.

        This is done with a qemu-img convert to raw/qcow2 from the snapshot
        qcow2.
        """

        LOG.debug("Copying snapshot: %(snap)s -> volume: %(vol)s, "
                  "volume_size: %(size)s GB",
                  {'snap': snapshot.id,
                   'vol': volume.id,
                   'size': volume_size})

        info_path = self._local_path_volume_info(snapshot.volume)
        snap_info = self._read_info_file(info_path)
        vol_path = self._local_volume_dir(snapshot.volume)
        forward_file = snap_info[snapshot.id]
        forward_path = os.path.join(vol_path, forward_file)

        # Find the file which backs this file, which represents the point
        # when this snapshot was created.
        img_info = self._qemu_img_info(forward_path, snapshot.volume.name)
        path_to_snap_img = os.path.join(vol_path, img_info.backing_file)

        path_to_new_vol = self._local_path_volume(volume)

        LOG.debug("will copy from snapshot at %s", path_to_snap_img)

        if self.configuration.nfs_qcow2_volumes:
            out_format = 'qcow2'
        else:
            out_format = 'raw'

        image_utils.convert_image(path_to_snap_img,
                                  path_to_new_vol,
                                  out_format,
                                  run_as_root=self._execute_as_root)

        self._set_rw_permissions_for_all(path_to_new_vol)
~~~

This function:
~~~
    def _set_rw_permissions_for_all(self, path):
        """Sets 666 permissions for the path."""
        self._execute('chmod', 'ugo+rw', path,
                      run_as_root=self._execute_as_root)
~~~

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/734343