moss: Add ability to disable repositories

[tarkah]

[tarkah]

hold on, won't this require us overwriting a vendor shipped repo configuration in /usr ... ? which would be overwritten by updates..?

We only write updates to admin /etc or user config dir. I'll test

[tarkah]

@ikeycode I've validated this works as expected:

• If we ship default repo / volatile to vendor path (relative to -D root) /usr/share/moss/repo.d, it shows up in moss lr
• User wants to "override" this / disable the repo: moss dr volatile
• Moss writes the overwrite to admin (relative to -D root) at /etc/moss/repo.d/volatile.yaml w/ active: false
• moss lr shows it disabled, since it always merges admin configs over vendor configs and we define repos as a hashmap, so same key name from admin merges over vendor