Don't set up sources in the refresh method of the Source spoke

Use the current source to set up the GUI elements in the refresh method of
the Source spoke, but don't change it. The source should be changed only in
the apply method.

Set up the spoke for all supported types of sources and raise an exception
if the current type is unsupported.

pyanaconda/ui/gui/spokes/installation_source.py

@@ -55,7 +55,6 @@
 from pyanaconda.modules.common.constants.objects import DEVICE_TREE
 from pyanaconda.modules.common.structures.storage import DeviceData
 from pyanaconda.core.storage import device_matches
-from pyanaconda.ui.lib.payload import set_source, tear_down_sources, create_source
 
 from pyanaconda.anaconda_loggers import get_module_logger
 log = get_module_logger(__name__)
@@ -973,20 +972,17 @@ def refresh(self):
                 self._iso_chooser_button.set_use_underline(False)
         elif source_type == SOURCE_TYPE_HMC:
             self._hmc_button.set_active(True)
-        else:
-            # No method was given in advance, so now we need to make a sensible
-            # guess.  Go with autodetected media if that was provided, and then
-            # fall back to closest mirror.
+        elif source_type == SOURCE_TYPE_CDROM:
+            # Go with autodetected media if that was provided,
+            # otherwise fall back to the closest mirror.
             if not self._autodetect_button.get_no_show_all():
                 self._autodetect_button.set_active(True)
-                source_type = SOURCE_TYPE_CDROM
             else:
                 self._network_button.set_active(True)
-                source_type = SOURCE_TYPE_CLOSEST_MIRROR
-
-            tear_down_sources(self.payload.proxy)
-            source_proxy = create_source(source_type)
-            set_source(self.payload.proxy, source_proxy)
+        elif source_type == SOURCE_TYPE_CLOSEST_MIRROR:
+            self._network_button.set_active(True)
+        else:
+            ValueError("Unsupported source type: '{}'".format(source_type))
 
         self._setup_updates()