PXE server credential validation endpoint

[skateman]

There's no endpoint for validating PXE server credentials. This is how it's done currently in the UI:

  def pxe_server_async_cred_validation
    begin
      PxeServer.verify_depot_settings(params[:pxe])
    rescue StandardError => bang
      render :json => {:status => 'error', :message => _("Error during 'Validate': %{error_message}") % {:error_message => bang.message}}
    end

    render :json => {:status => 'success', :message => _('PXE Credentials successfuly validated')}
  end

@miq-bot add_label help wanted