Prior to this commit, Vagrant would prompt for smb username and password every time, even if only smb_username was defined. This commit changes that by allowing a "default" username from the Vagrantfile, with the option of overriding it.
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.