Always honor syslog environment variables

The library allows you to configure logging to syslog with
SPOTIFY_SYSLOG_HOST and SPOTIFY_SYSLOG_PORT, but these would only
be honored if --syslog, --syslogHost or --syslogPort was passed in
on the command line. This commit makes it so that the environment
variables will be used even if the command line args are not specified.
This makes it easier to use within docker containers, so users can
simply pass the environment variables to the container, without
worrying about the other arguments.