Consider making uri in buildmanager $config['subtrees']['myproject']['uri'] accept arrays.

[bryanhirsch]

Having uri be a string works for teams collaborating on GitHub because all approved contributors use the same 'git' user: git@github.com:<some-user>/<project>.git.

But for projects maintained on drupal.org, maintainers each need to be able to push using their own approved user like this: <my-user>@git.drupal.org:project/<project>.git

Seems like we should make uri accept an array (bulleted list). If Drush Subtree discovers multiple options, it should prompt user to select which one to push to.