Implement defaults for config options

[AdamWill]

As reported downstream, at one point our openQA zezere test started failing. We run our own zezere server for the test. What happened is that zezere added a new config option, and the config file we use in the test did not include this option at all. The most common way server software would usually handle this situation is to use some kind of in-built default value, but zezere doesn't do this: if a config option isn't present in the config file, it just fails (ultimately because ConfigParser.getboolean() fails in settings_external.py).

Ideally this mechanism should be tweaked somehow so zezere can set and use defaults for config options that aren't set in the config file at all, then adding a new config option would not require server admins to update their config files unless they need to set it to something non-default.

[nullr0ute]

@rdotjain could you take a look at this?

[rdotjain]

As mentioned downstream, we provide default values in default.conf file, but it appears that this file isn't included in the zezere-ignition package. Therefore, we could either add that file to the package or provide a fallback value in the settings itself.

For example, SESSION_COOKIE_SECURE = getboolean("global", "secure_cookie", "SECURE_COOKIE", fallback=True)

[nullr0ute]

We can add the file to the package easily enough, where does it reside?

[AdamWill]

I'd note that just shipping a single monolithic config file also has problems (e.g. if someone edits it, they will never get future updates to it).

I don't know whether you've done this already or not, but if you want to solve it that way, I'd recommend looking into the systemd approach to config files (with snippets, and with the app's defaults shipped in /usr but overrideable from /etc or ~/.config).