As a user, I can upload modular content
Ticket moved to GitHub: "pulp/pulp_rpm/2229":https://github.com/pulp/pulp_rpm/issues/2229
A user should be able to upload a module metadata YAML file to Pulp, which will create modulemd and modulemd-default content units.
When new modules are created, they should be automatically associated with any referenced RPMs already present or which are added in the future.
In case modulemd-deafults, there is restriction that only one modulemd-defaults can exist for a repository, so we will either need to have a total replacement strategy, a merge strategy like with Errata, or require the user to delete the existing ones first. Validation will be very important.