fix bug with repositorypath not honoring --prefix
only reinitialize 'repository' if it is a repository already
also set prefix configuration option in unit test setup
unset $EASYBUILD_PREFIX for config unit tests
fix forcing reinitialisation of repository in options.py rather than in config.py
add unit test to check whether generaloption-style configuration overrides legacy style
fix config.py unit test by not unsetting $EASYBUILDPREFIX too early
Merge pull request #1031 from boegel/fix_repopath_prefix

fix issue with --repositorypath not honoring --prefix