heat metadata : remove metadata_url logic

heat metadata : remove metadata_url logic

Remove logic where the heat-metadata server registers a
URL on startup with the engine.  The instance metadata is
now served via the CFN api, so we just have a config-file
option specifying the URL of the CFN API.  We don't want to
preserve the "register on startup" logic, because we need the
engine to have access to this information even if it is
restarted independent of some other service (avoid reliance on
services coming up in a particular order)

Change-Id: I690170977227ec96451d2a2fd25f7e507370b604
Signed-off-by: Steven Hardy <shardy@redhat.com>