Prior to this commit, the puppet provisioner would use the manifest dir flag when running `puppet apply`. Not only is this flag redundant due to how puppet apply works, but it is also deprecated in Puppet 4 and removed in Puppet 5. This commit simply removes the flag when invoking `puppet apply`.
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.