FIX: CVMFS_KEYS_DIR Cannot be Unset

Dave pointed out, that there is no way to use the config variable `CVMFS_PUBLIC_KEY` since it is always overwritten by `CVMFS_KEYS_DIR` which cannot be unset. This also checks for an empty `CVMFS_KEYS_DIR` and ignores it in favour of `CVMFS_PUBLIC_KEY`.