Preserve preferred-challenges on renewal (#4112)

* use challenge type strings, not objectS

* Factor out parse_preferred_challenges

* restore pref_challs

* save pref_challs

* Make CheckCertCount more flexible

* improve integration tests

* Make pref_challs more flexible

certbot/auth_handler.py

@@ -34,8 +34,7 @@ class AuthHandler(object):
     :ivar list achalls: DV challenges in the form of
         :class:`certbot.achallenges.AnnotatedChallenge`
     :ivar list pref_challs: sorted user specified preferred challenges
-        in the form of subclasses of :class:`acme.challenges.Challenge`
-        with the most preferred challenge listed first
+        type strings with the most preferred challenge listed first
 
     """
     def __init__(self, auth, acme, account, pref_challs):
@@ -252,8 +251,10 @@ class AuthHandler(object):
         # Make sure to make a copy...
         plugin_pref = self.auth.get_chall_pref(domain)
         if self.pref_challs:
-            chall_prefs.extend(pref for pref in self.pref_challs
-                               if pref in plugin_pref)
+            plugin_pref_types = set(chall.typ for chall in plugin_pref)
+            for typ in self.pref_challs:
+                if typ in plugin_pref_types:
+                    chall_prefs.append(challenges.Challenge.TYPES[typ])
             if chall_prefs:
                 return chall_prefs
             raise errors.AuthorizationError(