PluginStorage to store variables between invocations. (#5468)

The base class for Installer plugins `certbot.plugins.common.Installer` now provides functionality of `PluginStorage` to all installer plugins. This allows a plugin to save and retrieve variables in between of invocations.

The on disk storage is basically a JSON file at `config_dir`/`.pluginstorage.json`, usually `/etc/letsencrypt/.pluginstorage.json`. The JSON structure is automatically namespaced using the internal plugin name as a namespace key. Because the actual storage is JSON, the supported data types are: dict, list, tuple, str, unicode, int, long, float, boolean and nonetype.

To add a variable from inside the plugin class:
`self.storage.put("my_variable_name", my_var)`

To fetch a variable from inside the plugin class:
`my_var = self.storage.fetch("my_variable_key")`

The storage state isn't written on disk automatically, but needs to be called:
`self.storage.save()`

* Plugin storage implementation

* Added config_dir to existing test mocks

* PluginStorage test cases

* Saner handling of bad config_dir paths

* Storage moved to Installer and not initialized on plugin __init__

* Finetuning and renaming

certbot/plugins/common.py

@@ -18,6 +18,8 @@ from certbot import interfaces
 from certbot import reverter
 from certbot import util
 
+from certbot.plugins.storage import PluginStorage
+
 logger = logging.getLogger(__name__)
 
 
@@ -99,7 +101,6 @@ class Plugin(object):
     def conf(self, var):
         """Find a configuration value for variable ``var``."""
         return getattr(self.config, self.dest(var))
-# other
 
 
 class Installer(Plugin):
@@ -110,6 +111,7 @@ class Installer(Plugin):
     """
     def __init__(self, *args, **kwargs):
         super(Installer, self).__init__(*args, **kwargs)
+        self.storage = PluginStorage(self.config, self.name)
         self.reverter = reverter.Reverter(self.config)
 
     def add_to_checkpoint(self, save_files, save_notes, temporary=False):