RPM Support

Support for relative-url in the export distributor config should come with #1394, but adding support for that to pulp-admin is a little tricky, warranting a brief discussion of the options:

Option 1:
Currently[0], --relative-url isn't passed into the export distributor config when repos are created or updated. If it was passed into the export distributor, it would probably result in a backward-incompatible change, since it will change the published repo path on the next publish and probably break anything depending on the location of that repo.

Option 2:
Breaking backward compatibility is an option if we want this for 3.0, but the safer option might be to add another flag just for the export distributer to the repo create/update command, e.g. --export-relative-url.

Option 3:
The export run command[1] supports its own config flags as runtime overrides, so --relative-url could be added to only that command, but this is a portentially crappy user experience (Why do I have to set relative-url again?) so again the --export-relative-url might be the flag to use.

Option 4 (my preference):
Some combination of options 1 and 2, and option 3: probably add --export-relative-url to repo create/update and also to export run, maintaining backward compatibility the whole way.

Option 5:
Do nothing. The option is in the docs, leave it to adventurous users to modify the export distributor config via the REST API.

Option 6:
Something I didn't think of.

I think this becomes a relatively easy task once an option is chosen.

[0]: https://github.com/pulp/pulp_rpm/blob/c95bf5af25173abce724261cdef5bb5509e613e0/extensions_admin/pulp_rpm/extensions/admin/repo_create_update.py
[1]: https://github.com/pulp/pulp_rpm/blob/c95bf5af25173abce724261cdef5bb5509e613e0/extensions_admin/pulp_rpm/extensions/admin/export.py