Allow specifying repositories manually

[supakeen]

image-builder currently uses repositories from a well-known path and these repositories can be adjusted there.

It'd be nice if I could also override them from the command line options and/or by pointing directly at a file without overriding all of them.

Alternatively we can document how to do this through JSON and the --datadir option.

This is a common thing to do in build systems (for example koji).

[mvo5]

This #113 should address this hopefully

[supakeen]

This #113 should address this hopefully

It does, in combination with overriding from JSON (though I'd still like both --extra-build-repo to add and --build-repo to override).