Fixes #37627 - Only update repositories once in global registration

[ekohl]

Rather than doing this for each repository, it is now performed once.

It also simplifies writing out the repo file to make the write atomic. Right now it doesn't use save_to_file, but that could simplify it even further.

[nadjaheitmann]

Test failure does not look related to the change, does it?

[nadjaheitmann]

Is there anything that hold this one back? @ekohl @sbernhard

[sbernhard]

[test foreman]