Script plugin (#3521)

* Script plugin initial commit

* Fix auth script path

* Return correct responses

* Added DNS-01 support

* Report the challenge pref correctly

* Use config root from certbot constants rather than hardcoded

* Remove prehook and rename posthook to cleanup for clarity

* Refactoring

* Docs

* Refactoring

* Refactoring continued, working now

* Use global preferred-challenges argument in favor of local

* Added http-01 as fallback challenge if not defined

* Do not continue if auth script not defined

* Skip unnecessary steps when running

* Read config values from correct places

* Tests and minor fixes

* Make Python 2.6 happy again

* Added CERTBOT_AUTH_OUTPUT and better tests

* Lint & Py3 fixes

* Make Python 2.6 happy again

* Doc changes

* Refactor hook execute and reuse in script plugin

* Refactored hook validation

* Added long_description for plugin help text

* Refactored env var writing

certbot/cli.py

@@ -80,6 +80,7 @@ USAGE = SHORT_USAGE + """Choice of server plugins for obtaining and installing c
   --standalone      Run a standalone webserver for authentication
   %s
   --webroot         Place files in a server's webroot folder for authentication
+  --script          User provided shell scripts for authentication
 
 OR use different plugins to obtain (authenticate) the cert and then install it:
 
@@ -92,7 +93,7 @@ More detailed help:
 
    all, automation, paths, security, testing, or any of the subcommands or
    plugins (certonly, renew, install, register, nginx, apache, standalone,
-   webroot, etc.)
+   webroot, script, etc.)
 """
 
 
@@ -587,7 +588,8 @@ class HelpfulArgumentParser(object):
 
         """
         for name, plugin_ep in six.iteritems(plugins):
-            parser_or_group = self.add_group(name, description=plugin_ep.description)
+            parser_or_group = self.add_group(name,
+                                             description=plugin_ep.long_description)
             plugin_ep.plugin_cls.inject_parser_options(parser_or_group, name)
 
     def determine_help_topics(self, chosen_topic):
@@ -989,6 +991,8 @@ def _plugins_parsing(helpful, plugins):
                 help="Obtain and install certs using Nginx")
     helpful.add(["plugins", "certonly"], "--standalone", action="store_true",
                 help='Obtain certs using a "standalone" webserver.')
+    helpful.add(["plugins", "certonly"], "--script", action="store_true",
+                help='Obtain certs using shell script(s)')
     helpful.add(["plugins", "certonly"], "--manual", action="store_true",
                 help='Provide laborious manual instructions for obtaining a cert')
     helpful.add(["plugins", "certonly"], "--webroot", action="store_true",