Unset the imageRef when booting from volume

When testing boot from volume, we were passing the image_ref no matter
what to servers.create. This will cause nova to use the metadata from
that image instead of the one that might contain the volume.

This patch sets the image to an empty string, which is the same
behaviour the nova CLI has when booting from a volume.

Change-Id: I59bd4e2be6d6fff782dd278390ad5241cd09e3df
Fix V3 image tests according to new image_client

This patch correct the Nova V3 test_image_oneserver
and test_images_negative according to new image_client.
Also remove the create_image config value.

Closes-Bug: #1290221

Change-Id: Iadaf311e37a42b2adaa9d980a10ae251450030d0
Merge "Fix V3 image tests according to new image_client"
Merge "Unset the imageRef when booting from volume"
Merge branch 'master' of https://github.com/openstack/tempest