repo: Add add-remotes-config-dir option

repo: Add add-remotes-config-dir option

This option allows a repo to explicitly opt out of adding new remotes in
a remotes configuration directory. This currently defaults to true for
system repos and false for non-system repos to maintain legacy behavior
that non-system repos don't add remotes in a configuration directory.
That would be problematic for flatpak, which specifies a remotes config
dir but adds remotes in ways that are incompatible with it.

So, what this really does is allow system repos to control whether they
want to add remotes in the config dir or not. That's important if your
flatpak repo is the system repo like at Endless.

Closes: #1134
Closes: #1155
Approved by: cgwalters