[systemd,logs] move journalctl logs to logs plugin

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>
[logs] improve log collection for journald-only configurations

If no syslog exists collect the last three days worth of
journald logs by default and allow the user to specify a "log_days"
option to control the data collected.

Fixes #325.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>