(PUP-1596) Expand all paths when creating directory environments

When reading EnvironmentConf settings, relative paths were prepended
with the directory environment path. This commit ensures that we also
expand the paths properly so that they are absolute on both Windows and
Posix platforms.
Merge pull request #2460 from jpartlow/feature/master/pup-1596-expand-paths-for-env-on-windows

(PUP-1596) Expand all paths when creating environments