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>

TESTING

@@ -23,7 +23,7 @@ INSTALL.
     $ sqlite3 ../aurweb.sqlite3 < aur-schema-sqlite.sql
     $ sqlite3 ../aurweb.sqlite3 < out.sql
 
-4) Copy conf/config.proto to conf/config and adjust the configuration
+4) Copy conf/config.defaults to conf/config and adjust the configuration
    (pay attention to disable_http_login, enable_maintenance and aur_location).
 
    Be sure to change backend to sqlite and name to the file location of your