[3.0.1 backport] Console logging not being enabled when only a log key set

Problem

The behavior to enable console logging at info-level only when a log key is set is not working in 3.0

To enable console logging in 3.0, users must also specify a console log level, e.g. "log_level": "info"

repro

1. Start SG with a log key defined, but no other console log settings.
2. Observe there are no logs... Setting a log key should enable info logging.

proposed fix

DefaultStartupConfig should not be explicitly setting the Console log level to None to allow for the handling inside ConsoleLoggerConfig.Init()

https://github.com/couchbase/sync_gateway/blob/d77d1a69eee81bd8d888b17558cf454fe9b4606f/base/logger_console.go#L175-L177