Fix typo so setting volume_tmp_dir works

apply the same change that was applied to cinder
https://review.openstack.org/#/c/14863/

Fixes LP bug #1071536

Change-Id: I4cfdb38debfcb4a890c2d7fd7405ac9dc755f0ec
(cherry picked from commit 3f1af292f4771a384230ec80c32f2d253cef8bc1)

nova/volume/driver.py

@@ -845,7 +845,7 @@ def copy_image_to_volume(self, context, volume, image_service, image_id):
         # TODO(jdurgin): replace with librbd
         # this is a temporary hack, since rewriting this driver
         # to use librbd would take too long
-        if FLAGS.volume_tmp_dir and not os.exists(FLAGS.volume_tmp_dir):
+        if FLAGS.volume_tmp_dir and not os.path.exists(FLAGS.volume_tmp_dir):
             os.makedirs(FLAGS.volume_tmp_dir)
 
         with tempfile.NamedTemporaryFile(dir=FLAGS.volume_tmp_dir) as tmp: