Automatically download Drush dependencies

In #2224411: Declare dependencies on Drush modules, we're looking at adding a 'drush dependencies' array to Hosting features. In #4 of #2180053: Include more "golden" contrib, I suggested:

allow hosting features to declare the Drush extensions they require. Then, when we verify the hostmaster site to register enabled features, we could also download these extensions and run a 'drush cc drush'.

This issue is to follow-up on the latter, which obviously depends on the former. We should probably add a setting to allow control over automatically downloading Drush extensions. Presumably, we'll want to check whether they're already installed before trying to download them.

Also, I don't believe Drush is aware of the versions of the extensions that it is running (.info files are optional). So we'd probably need to track that ourselves. In that case, we may also need to declare dependency versions in our Hosting features.