Prior to this commit, if a user attempted to configure
`/etc/default/docker` through vagrant prior to installation, the package
manager would not override an existing configuration and installing
docker would then fail. This commit fixes this by introducing a
`post_install_provisioner` that allows users to define a provisioner
block that will run after docker has been installed, allowing users to
configure `/etc/default/docker` how they want.
Brian Cain