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.

https://bugs.launchpad.net/cinder/+bug/1288240

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Merge pull request #478 from enovance/volume_tmp_dir

cinder/rbd: configure volume_tmp_dir to /tmp