Default to info level for an empty log-level

I'd assumed that the omitempty directive for LogLevel in internal/config/config.go would get us this behaviour. If it did, we wouldn't have had to specify the default twice. Unfortunately, it doesn't, which is to say that given a config file like:

log_level:

The default is overridden by the empty string.

It's an easy enough fix, and especially for gitlab-shell (rather than gitlab-sshd), the message will otherwise appear once per connection, so definitely worth it.

Closes #550 (closed)