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>

INSTALL

@@ -40,8 +40,10 @@ read the instructions below.
 
     Ensure to enable the pdo_mysql extension in php.ini.
 
-3) Copy conf/config.proto to /etc/aurweb/config and adjust the configuration
-   (pay attention to disable_http_login, enable_maintenance and aur_location).
+3) Optionally copy conf/config.defaults to /etc/aurweb/. Create or copy
+   /etc/aurweb/config (this is expected to contain all configuration settings
+   if the defaults file does not exist) and adjust the configuration (pay
+   attention to disable_http_login, enable_maintenance and aur_location).
 
 4) Create a new MySQL database and a user and import the aurweb SQL schema: