Allow providing an explicit list of repos in shepherd.yml

[parshap]

Sometimes you have an exact list of repos you want to apply a migration to, it'd be nice if you could provide a list of repos in shepherd.yml instead of using a GitHub search.

[alexturek]

For future finders of this issue: one workaround is via CLI (discovered via shepherd checkout --help)

shepherd checkout --repos org1/repo1,org2/repo2 path/to/migration