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/plugins/selection.py

@@ -131,7 +131,7 @@ def choose_plugin(prepared, question):
         else:
             return None
 
-noninstaller_plugins = ["webroot", "manual", "standalone"]
+noninstaller_plugins = ["webroot", "manual", "standalone", "script"]
 
 def record_chosen_plugins(config, plugins, auth, inst):
     "Update the config entries to reflect the plugins we actually selected."
@@ -236,6 +236,8 @@ def cli_plugin_requests(config):
         req_auth = set_configurator(req_auth, "webroot")
     if config.manual:
         req_auth = set_configurator(req_auth, "manual")
+    if config.script:
+        req_auth = set_configurator(req_auth, "script")
     logger.debug("Requested authenticator %s and installer %s", req_auth, req_inst)
     return req_auth, req_inst