Next

This is way smaller than it seems.

The actual changes are:
* loose dev dependencies on docker-registry-core as well (already have them on main)
* introducing DEPS=loose environment variable that allows to install the main package itself with loose dependencies instead of strict one (this is meant for adventurous developers, package maintainers desires as in #466 ping @mika and fixes #509): to use simply `DEPS=loose pip install docker-registry`
* uniformized various gunicorn launch options (config file or through wrapper script), and deprecated the direct call to gunicorn

Now, loosing the dev dependencies broke on us (<- that's me paying for my sins listening to @wking chanting) - as hacking became more strict, so I fixed everything to make it happy.
This included changing the order of a number of imports - which always present a small risk of breaking, so, watch out for these.


The rest is really various trivial cleanup and documentation update.

@wking @shin- your (not necessarily) kind reviews expected here :-)