Drop pulp support and remove pulp support configuration and dependencies.

[nuclearsandwich]

This is a rebase of #118 with some modifications:

• Address Remove Pulp configuration #118 (comment) by removing the redis and postgresql packages (but does not "purge" them, removing configuration and data as well although I am tempted to do so).

• Address Remove Pulp configuration #118 (comment) and the test failure by re-enabling the docker-registry by default.

• Document that the pulp cleanup stanzas can be removed in the next major release.

[j-rivero]

Can not detect any big problem, just a check on a system path in a comment I left before.

[nuclearsandwich]

In addition to the CI checks, I also confirmed that this branch can be converged on top of 0.5.1 validating it for inclusion in a 0.6.0 release.

This also converges cleanly on 0.5.0 which means that we could technically release it in a 0.5.2 although since the scope of this change is to remove an entire feature (that was switched off by default in 0.5.1 I think a tick-tock approach across two major versions is warranted.