Clone specific repositories as well

This will add a functionality to clone specific repositories
from ownCloud organization.

It also generates an error if the core environment is not set up.

To use: `python3 -m ocdev setup <repositoryname>`

The conditions for base and core are added because otherwise
the setup command could be used only once
https://github.com/owncloud/ocdev/pull/1/files#diff-f8414077331652fc2d5829655cf9ad60R102