docker_registry.run: Restore GUNICORN_OPTS functionality

This was initially added in 534c6da5 (run.sh: Add '$GUNICORN_OPTS
"$@"' to gunicorn invocation, 2014-02-13), but didn't survive the
translation to a setuptools entry_points in bf3360a2 (replacing run.sh
with a "docker-registry" console script, 2014-04-02).  bf3360a2 was
part of dotcloud/docker-registry#297, which branched off before
534c6da5 landed with dotcloud/docker-registry#247, and the
entry_points invocation wasn't updated to reflect the change.

The syntax has also changed a bit, since we're invoking Gunicorn from
Python instead of Bash.  Instead of dropping the string verbatim onto
the command-line ($GUNICORN_OPTS), it now goes through the usual
docker_registry.server.env.source YAML parsing (884b418f, Type from
environment properly (like Config), 2014-06-23).  So instead of
something like:

  GUNICORN_OPTS='--preload --log-syslog'

you should now be using:

  GUNICORN_OPTS='[--preload, --log-syslog]'
Merge pull request #483 from wking/gunicorn-opts

docker_registry.run: Restore GUNICORN_OPTS functionality