Removed unncessary code

no need to call .keys()
if fixture_key not in reads more cleanly

Change-Id: I27108fb7c1b71cb7a328a35d82d79a9da4786171
Removed unnecessary code

Modified glance/registry/client.py.
Replaced image_metadata.keys() with image_metadata

Change-Id: I1e80ec476a8a23d9935ec9d111d652781cad320c