Implement user configuration file

Previously, it was recommended to use ~/.novaboot as user configuration
file, but this didn't work when working other directory, such as in
/tmp. Therefore a new configuration file ~/.config/novaboot is
introduced, which is read always when it exists.

Thanks to Michal Vokac for figuring out this problem.
Do not read user config during tests

This is to make the tests independent of settings of the user running
the tests.