Allow setting log levels via env var

[mikkonie]

We would probably want to allow different levels of logging for different server instances.

Note that iRODS 4.3 uses a dict with categories for logging config which has to be supported. Example:

{
    "log_level": {
        "agent": "info",
        "agent_factory": "info",
        "api": "info",
        "authentication": "info",
        "database": "info",
        "delay_server": "info",
        "legacy": "info",
        "microservice": "info",
        "network": "info",
        "resource": "info",
        "rule_engine": "info",
        "server": "info",
        "sql": "info"
    }
}

Related to #28, it would make sense to do these for the same release.

[mikkonie]

As a note, this has to be set up in a way where the level(s) can also be changed on an already installed server. So simply providing env var(s) to the unattended install template is not enough.