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.
Vagrant Core Plugins
These are plugins that ship with Vagrant. Vagrant core uses its own plugin system to power a lot of the core pieces that ship with Vagrant. Each plugin will have its own README which explains its specific role.