Added API tests for Docker images.

Added two new API tests for creating repositories that use Docker
content.

**NOTE**: To synchronize a ``Docker`` image, the ``name`` of your custom
  repository **must** match (case matters) the name of the Docker image.
  For example, the
  [WordPress](https://registry.hub.docker.com/_/wordpress/) image would
  require that your repository name be ``wordpress`` as it is displayed
  in its URL.
this import may simplify these statements:
```
from robottelo.common.decorators import data, run_only_on
```