fix configuration issues, in particular backwards compatibility + add config unit tests

This pull request fixes a couple of configuration problems introduced by #529, in particular backwards compatibility ('old-style' configuration), and the use of `--prefix` that resulted in `KeyError` exceptions.

A couple of unit tests that mainly test the default configuration using `$HOME/.local/easybuild` and legacy configuration options (both environment variables like `$EASYBUILDPREFIX` and co and config files `$HOME/.easybuild/config.py` and using `$EASYBUILDCONFIG`).