[autotest] Privatize sort_actionable_labels()

BUG=chromium:707999
TEST=Run unit tests

Change-Id: I85d16ac1238b0b9f5a6b85041f2f0ca611307872
Reviewed-on: https://chromium-review.googlesource.com/446886
Commit-Ready: Allen Li <ayatane@chromium.org>
Tested-by: Allen Li <ayatane@chromium.org>
Reviewed-by: Allen Li <ayatane@chromium.org>
diff --git a/server/cros/provision.py b/server/cros/provision.py
index 3a4fd16..f74af58 100644
--- a/server/cros/provision.py
+++ b/server/cros/provision.py
@@ -177,7 +177,7 @@
                        "Can't %s label '%s'." % (cls.name, label))
 
         # Sort the configuration labels based on `cls._priorities`.
-        sorted_actionable = cls.sort_actionable_labels(actionable)
+        sorted_actionable = cls._sort_actionable_labels(actionable)
         for name, value in sorted_actionable:
             action_item = cls.action_for(name)
             success = action_item.execute(job=job, host=host, value=value)
@@ -226,7 +226,7 @@
 
 
     @classmethod
-    def sort_actionable_labels(cls, labels):
+    def _sort_actionable_labels(cls, labels):
         """
         Sort configurations based on the priority defined in cls._priorities.