Ability to specify a default provider

[bspaans]

For exampe in the new docker-builder release we're consuming a docker-repository provider. It would be nice if could default that to the default docker-repository-provider.

[bspaans]

Not sure if this is such a great idea. You would also need to enable input mappings and whatnot... starts to defeat the purpose of the provider a bit perhaps? Maybe this use case would be better served with optional providers