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

setup.py

@@ -132,6 +132,7 @@ setup(
             'null = certbot.plugins.null:Installer',
             'standalone = certbot.plugins.standalone:Authenticator',
             'webroot = certbot.plugins.webroot:Authenticator',
+            'script = certbot.plugins.script:Authenticator',
         ],
     },
 )