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/cli.py

@@ -1,4 +1,5 @@
 """Certbot command line argument & config processing."""
+# pylint: disable=too-many-lines
 from __future__ import print_function
 import argparse
 import copy
@@ -1236,13 +1237,31 @@ class _PrefChallAction(argparse.Action):
     """Action class for parsing preferred challenges."""
 
     def __call__(self, parser, namespace, pref_challs, option_string=None):
-        aliases = {"dns": "dns-01", "http": "http-01", "tls-sni": "tls-sni-01"}
-        challs = [c.strip() for c in pref_challs.split(",")]
-        challs = [aliases[c] if c in aliases else c for c in challs]
-        unrecognized = ", ".join(name for name in challs
-                                 if name not in challenges.Challenge.TYPES)
-        if unrecognized:
-            raise argparse.ArgumentTypeError(
-                "Unrecognized challenges: {0}".format(unrecognized))
-        namespace.pref_challs.extend(challenges.Challenge.TYPES[name]
-                                     for name in challs)
+        try:
+            challs = parse_preferred_challenges(pref_challs.split(","))
+        except errors.Error as error:
+            raise argparse.ArgumentTypeError(str(error))
+        namespace.pref_challs.extend(challs)
+
+
+def parse_preferred_challenges(pref_challs):
+    """Translate and validate preferred challenges.
+
+    :param pref_challs: list of preferred challenge types
+    :type pref_challs: `list` of `str`
+
+    :returns: validated list of preferred challenge types
+    :rtype: `list` of `str`
+
+    :raises errors.Error: if pref_challs is invalid
+
+    """
+    aliases = {"dns": "dns-01", "http": "http-01", "tls-sni": "tls-sni-01"}
+    challs = [c.strip() for c in pref_challs]
+    challs = [aliases.get(c, c) for c in challs]
+    unrecognized = ", ".join(name for name in challs
+                             if name not in challenges.Challenge.TYPES)
+    if unrecognized:
+        raise errors.Error(
+            "Unrecognized challenges: {0}".format(unrecognized))
+    return challs