standalone: do not attempt webdir challenge verification

webdir challenge verfication will always fail with standalone due to the
server not being started immediately at the point of challenge creation.

acertmgr/modes/standalone.py

@@ -63,6 +63,7 @@ HTTPServer.allow_reuse_address = True
 class ChallengeHandler(WebChallengeHandler):
     def __init__(self, config):
         WebChallengeHandler.__init__(self, config)
+        self._verify_challenge = False
         self.current_directory = os.getcwd()
         if "port" in config:
             port = int(config["port"])

acertmgr/modes/webdir.py

@@ -14,6 +14,7 @@ from acertmgr.modes.abstract import AbstractChallengeHandler
 class ChallengeHandler(AbstractChallengeHandler):
     def __init__(self, config):
         AbstractChallengeHandler.__init__(self, config)
+        self._verify_challenge = True
         self.challenge_directory = config.get("webdir", "/var/www/acme-challenge/")
         if not os.path.isdir(self.challenge_directory):
             raise FileNotFoundError("Challenge directory (%s) does not exist!" % self.challenge_directory)
@@ -30,6 +31,7 @@ class ChallengeHandler(AbstractChallengeHandler):
 
         # check that the file is in place
         wellknown_url = "http://{0}/.well-known/acme-challenge/{1}".format(domain, token)
+        if self._verify_challenge:
             try:
                 resp = tools.get_url(wellknown_url)
                 resp_data = resp.read().decode('utf8').strip()