API Changes

[dralley]

The schema looks like this:

"_content": {
    "file": "/pulp/api/v3/content/file/files/?repository_version=..."
},
"_content_added": {
    "file": "/pulp/api/v3/content/file/files/?repository_version_added=..."
},
"_content_removed": {}

Change the get_*content() functions to return a dictionary of types
matched to the respective list of content of that type.

Fix the get_artifact_paths() function to use those new functions
properly.

[nixocio]

Those changes were tested with the new API layout.

See: https://github.com/pulp/pulp_file/pull/133/files

[nixocio]

@dralley , do you have any other change do add to this PR?

Tests passed in the other repo, if this is PR blocking the Pulp release, we can go ahead and merge this PR.

[dralley]

@kersommoura No others, but please review these. That way we can merged them all together to minimize the amount of breakage.

pulp/pulp#3774
pulp/pulp_file#133
pulp/pulp_rpm#1248

[nixocio]

@dralley, I reviewed those changes related to the functional tests. I left minor comments.