config: allow reading both the defaults file and the modified config

In the process, rename config.proto to config.defaults (because that is
what it is now).

Also use dict.get('key', default_value) when querying os.environ, rather
than an if block, as it is more pythonic/readable/concise, and reduces
the number of dict lookups.

This change allows aurweb configuration to be done via either:
- copying config.defaults to config and modifying values
- creating a new config only containing modified values, next to a
  config.defaults containing unmodified values

The motivation for this change is to enable ansible configuration in our
flagship deployment by storing only changed values, and deferring to
config.defaults otherwise.

A side benefit is, it is easier to see what has changed by inspecting
only the site configuration file.

If a config.defaults file does not exist next to $AUR_CONFIG or in
$AUR_CONFIG_DEFAULTS, it is ignored and *all* values are expected to
live in the modified config file.

Signed-off-by: Eli Schwartz <eschwartz@archlinux.org>
Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>

aurweb/config.py

@@ -8,11 +8,13 @@ def _get_parser():
     global _parser
 
     if not _parser:
+        path = os.environ.get('AUR_CONFIG', '/etc/aurweb/config')
+        defaults = os.environ.get('AUR_CONFIG_DEFAULTS', path + '.defaults')
+
         _parser = configparser.RawConfigParser()
-        if 'AUR_CONFIG' in os.environ:
-            path = os.environ.get('AUR_CONFIG')
-        else:
-            path = "/etc/aurweb/config"
+        if os.path.isfile(defaults):
+            with open(defaults) as f:
+                _parser.read_file(f)
         _parser.read(path)
 
     return _parser