Config: Change env to lower priority than config file.

[winlinvip]

Current config priority:

1. Low: Default values.
2. Medium: Config file.
3. High: Environment variables.

If config is set in dockerfile, user can only overwrite it by docker run --env, but not by config file.

I think the priority should be:

1. Low: Default values.
2. Medium: Environment variables.
3. High: Config file.

So as to change and reload by config file.

[winlinvip]

You can configure SRS by config file and env. I think the priority is not the key point. This means it's acceptable when config file has a higher priority than env.