Create new repo version adding or removing content.

[nixocio]

Create test to explore the design choice described 3234

Possible steps to test.

1 . Add content to Pulp - using sync to a repoA.
2. Create a second repoB.
3. Use add_content_units to pass a list of urls for units to be added to repoB.
4. Assert that units are present on repoB.
5. Assert that version of repoB has changed.