RPM Support

Storing the SLES auth token is pretty trivial, since pulp already makes it easy to tweak importer configs via the REST API. Here's an HTTPie example:

http --auth admin:admin --verify no PUT https://localhost/pulp/api/v2/repositories/sles-12/ importer_config:='{"sles_auth_token": "foo"}'

Building the URL modifier is easy enough, and I'm also able to DRY up some code, since pulp_rpm does a bit of trailing slash checking, appending stuff to url paths, etc, that benefits from centralizing the rewriting behavior.

I believe I've wired up the URL rewriting where needed to sync SLES 12+ repos, but writing the tests to prove it is proving a little bit tricky, most likely due to my lack of experience with Mock. :)

sles_auth_token became the more generic query_auth_token, so the httpie example above would turn into

http --auth admin:admin PUT https://host/pulp/api/v2/repositories/repo-id/ importer_config:='{"query_auth_token": "foo"}'

https://github.com/pulp/pulp_rpm/pull/734 was superseded by https://github.com/pulp/pulp_rpm/pull/736