Windows Powershell commands need to specify shell

The WinRM communicator defaults to the regular Windows cmd shell so we need to tell Packer to use Powershell when running these guest commands.

provisioner/guest_commands.go

@@ -23,8 +23,8 @@ var guestOSTypeCommands = map[string]guestOSTypeCommand{
 	},
 	WindowsOSType: guestOSTypeCommand{
 		chmod:     "echo 'skipping chmod %s %s'", // no-op
-		mkdir:     "New-Item -ItemType directory -Force -ErrorAction SilentlyContinue -Path %s",
-		removeDir: "rm %s -recurse -force",
+		mkdir:     "powershell.exe -Command \"New-Item -ItemType directory -Force -ErrorAction SilentlyContinue -Path %s\"",
+		removeDir: "powershell.exe -Command \"rm %s -recurse -force\"",
 	},
 }