Cache: Add method to completely refresh the cache

When grabbing an instance to the apt wrapper handle, currently it will
always return the same cache instance and will never fully refresh. This
can cause a problem when the underlying sources change as it will not
re-read these when it updates. Add a method to refresh this and utilise
for the Stretch upgrade.

kano_updater/apt_wrapper.py

@@ -83,6 +83,10 @@ def __init__(self):
         else:
             AptWrapper._singleton_instance = self
 
+        self._cache = None
+        self.refresh_instance()
+
+    def refresh_instance(self):
         apt.apt_pkg.init_config()
 
         # We disable downloading translations, because we don't have them

kano_updater/scenarios.py

@@ -1048,6 +1048,7 @@ def beta_3_15_0_to_beta_3_16_0(self, progress):
         # Proceeding to update to 4.x.x - add new sources and relaunch
         shutil.copy(REFERENCE_STRETCH_LIST, STRETCH_MIGRATION_LIST)
         apt_handle = AptWrapper.get_instance()
+        apt_handle.refresh_instance()
         apt_handle.update(progress)
         progress.relaunch()