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
No, it gives:

~~~
python3 -m ocdev setup ocdev

Selected setup type is neither core nor base. Trying to
clone repository named ocdev.
Failed to clone repository because Git is not installed
~~~
Because basically we do the same handling for subprocess as FileNotFound error.