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
Yes, but if i run git setup core it also runs into that branch https://github.com/owncloud/ocdev/blob/clonespecific/ocdev/plugins/setup/setup.py#L140