This commit introduces the salt_arg option that allows a user to pass additional command line flags to the `salt` tool when provisioning with a master setup. It also adds additional config validation to ensure that both `salt_args` and `salt_call_args` is an array.
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.