Ignore Powershell progress updates on stderr

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.

plugins/communicators/winrm/communicator.rb

@@ -212,10 +212,11 @@ module VagrantPlugins
           file.unlink
         end
 
-        # convert to double byte unicode string then base64 encode
-        # just like PowerShell -EncodedCommand expects
+        # Convert to double byte unicode string then base64 encode
+        # just like PowerShell -EncodedCommand expects.
+        # Suppress the progress stream from leaking to stderr.
         wrapped_encoded_command = Base64.strict_encode64(
-          "#{command}; exit $LASTEXITCODE".encode('UTF-16LE', 'UTF-8'))
+          "$ProgressPreference='SilentlyContinue'; #{command}; exit $LASTEXITCODE".encode('UTF-16LE', 'UTF-8'))
 
         "powershell -executionpolicy bypass -file \"#{guest_script_path}\" " +
           "-username \"#{shell.username}\" -password \"#{shell.password}\" " +

plugins/communicators/winrm/scripts/elevated_shell.ps1.erb

@@ -11,6 +11,7 @@ Try {
   exit $LASTEXITCODE
 }
 
+$ProgressPreference = "SilentlyContinue"
 $task_name = "WinRM_Elevated_Shell"
 $out_file = "$env:SystemRoot\Temp\WinRM_Elevated_Shell.log"
 

plugins/communicators/winrm/shell.rb

@@ -54,8 +54,10 @@ module VagrantPlugins
       end
 
       def powershell(command, &block)
-        # ensure an exit code
+        # Suppress the progress stream from leaking to stderr
+        command = "$ProgressPreference='SilentlyContinue';\r\n" + command
         command << "\r\n"
+        # Ensure an exit code
         command << "if ($?) { exit 0 } else { if($LASTEXITCODE) { exit $LASTEXITCODE } else { exit 1 } }"
         execute_shell(command, :powershell, &block)
       end