The hacky `minion_id` method to pass `file-root` and `pillar-root` to
`salt-call` does not work since Vagrant 1.8.3.

If Vagrant accepts hashicorp/vagrant#7836 , this PR will use the new
argument. It also sets the hostname using the appropriate option,
superseeding #248.

DO NOT MERGE until Vagrant accepts the PR, and releases a new version.