Debian Package for CERN Public Keys

This removes the *cvmfs-keys* package from the usual nightly build process of debian packages and introduces an extra build procedure for a stand alone CERN Public Keys debian package.

**Note:** This Pull Request includes some beautifications of the `CMakeLists.txt` files and is therefore a bit bloated. I mainly replaced all tab stops by two spaces to make them consistent. 

(work in progress... I hope it works on the build machine :o) )