Don't require any configuration

By default, FLASK fails if the env var passed to `app.config.from_envvar` isn't set.  But relengapi should run reasonably well with no explicit configuration.

While I'm at it, validate.sh should also unset `RELENGAPI_SETTINGS` when running relengapi.
nit: move "using default settings" to here, and use something that calls out the env_var, like `logger.warning("using default settings, to change set %s to the path to your settings file", env_var)`

Not too picky on wording of course.