Description of problem:
At the last step of auto attach a subscription with subscription-manager-gui, the "Attach" button should not show.

Version-Release number of selected component (if applicable):
subscription-manager-gui-1.15.9-11.el7.x86_64
subscription-manager-1.15.9-11.el7.x86_64
subscription-manager-initial-setup-addon-1.15.9-11.el7.x86_64
python-rhsm-1.15.4-5.el7.x86_64

How reproducible:
always

Steps to Reproduce:
1.Prepare a system that only registered and not have a subscription.
2.open subscription-manager-gui with cmd
#subscription-manager-gui
3.press the "Auto-attach" button in the "My Installed Products" to start the auto attach progress
4.Press "Attach" button when it find a proper subscription to attach.


Actual results:
While the attaching progress, the "Attach" button is still available,and when you press it, you'll get a traceback in the terminal that you opened the subscription-manager-gui like below:

[root@dhcp-129-146 rhsm]# subscription-manager-gui
Gtk-Message: GtkDialog mapped without a transient parent. This is discouraged.
Traceback (most recent call last):
  File "/usr/share/rhsm/subscription_manager/gui/registergui.py", line 420, in _on_proceed
    self.apply_current_screen()
  File "/usr/share/rhsm/subscription_manager/gui/registergui.py", line 424, in apply_current_screen
    self._screens[self._current_screen].apply()
  File "/usr/share/rhsm/subscription_manager/gui/registergui.py", line 719, in apply
    self.emit('move-to-screen')
TypeError: 1 parameters needed for signal move-to-screen; 0 given


Expected results:
The attach button should be grey or not showed.

Additional info: