cinder/rbd: configure volume_tmp_dir to /tmp

Set default temporary volume directory in rbd driver to /tmp, otherwise
we can hit some bugs in Cinder where it would be not possible to copy a
volume to an image.

It will also fix this broken test:
tempest.api.volume.test_volumes_get.VolumesGetTestXML.test_volume_create_get_update_delete_from_image
https://bugs.launchpad.net/cinder/+bug/1288240