Log to the system logger

[clcollins]

SREPD should log to the system logger on any systems it supports, and which have an accessible system logger, eg: the journal for Linux/Systemd. It looks like coreos/go-systemd/journal and even supports Windows, not just Linux & Unix (eg Mac) , supports this pretty easily, but the logging currently in SREPD would need to be adjusted somewhat substantially, particularly if we want to keep the charm logging format.

Done Criteria:

Logging to ~/.config/srepd/debug.log no longer occurs, and logs all end up in the system log (Systemd's journal, or otherewise).