Use Tempest-lib's compute images_client

Compute images_client has been migrated to
tempest-lib- I495915a5a40f7d351cb69463c032281e02c907c7

Now Tempest-lib provides this client as stable interface so Tempest
should start using this from lib and remove its own copy.
Also remove its unit tests.

This commits makes Tempest to use compute images_client
from Tempest-lib.

Change-Id: I4a32fed401251a26081a052f009d9cbee86ed11f
4 files changed