774940521e
Starting with PowerShell 5, the progress bar can be observed via the Write-Progress cmdlet. From WinRM, this appears as a stderr output. Vagrant assumes that there is error if output appears on stderr. This terminates various scripts which previously executed successfully in Vagrant (prior to Windows 10). This fix injects a variable assignment at various points of the script execution process to disable display of the progress bar.
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.