Add option to ignore default config files

[Flamefire]

We should have an option (and env variable) like --disable-default-configfiles to not load config files from the default locations such as $HOME, $XDG_DATA_HOME, $XDG_CONFIG_DIRS

The trouble/problem I'm trying to solve: We have a build script for the cluster which uses --configfiles=global.cfg such that the EB configuration will be the same/as specified. But now a EB config in the users HOME or in one of the other default paths may still be used setting stuff that breaks the builds.

This is more troublesome as it isn't just a single folder in $HOME (which could be excluded automatically via --ignoreconfigfiles) but actually includes locations derived from lists of paths stored in multiple XDG env variables. E.g. $XDG_CONFIG_DIRS: /etc/xdg/xdg-cinnamon:/etc/xdg already adds 2(!) more locations searched for the EB config

Note: Care must be taken such that --show-config continues to work, as that AFAIK reparses the config with different config sources disabled and hence could accidentally disable this new option and show wrong results.