Fix docker container linking

v1.6.0 assumed (rightfully) that driver's `params[:links]` is a hash. Then b68cb6d started to treat this param as an array instead (wrongfully). It was included in v1.6.1, which tries to run something like this `docker run ... --link ["db", "db"]` instead of `--link "db:db"`.

`params[:links]` is [indeed](https://github.com/mitchellh/vagrant/blob/master/plugins/providers/docker/action/create.rb#L92-96) a hash and not an array.

This PR reverts that commit and fixes the tests to pass a hash and not an array to `Driver#create`.