Previously, there was no one gesture that would start a VM if it was not running and run the appropriate provisioners regardless of its original state. `vagrant up` did nothing if the VM was running, while `vagrant provision` did nothing if the VM was not running. Change the semantics of `vagrant up`, via the start actions of the providers, to go through the provisioning logic even if the VM is already running. The semantics of `run: "once"` vs `run: "always"` are respected. Tested with the VirtualBox provider but not the others. Resolves #4421
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.