Correct image_metadata API use of nova.image.glance

The image_metadata API controller was using the nova.image.glance module
directly. This patch cleans that up and removes the associated unit
test's use of stub_out_glance() from the nova.tests.api.openstack.fake
module.

Change-Id: Ia4a6cc54600c2154e14adb43184ac4ebfb7de539
Merge "Correct image_metadata API use of nova.image.glance"