Added rsyslogd configuration for logging.

tasks/main.yml:
- Added task for configuring rsyslogd when redis_syslog_enabled ==
  "yes".
templates/redis.conf.j2:
- logfile is only configured if not using syslog;
- Syslog facility is parametrized.
template/rsyslogd.conf.j2:
- rsyslogd conf for filtering redis logs to redis_logfile.
defaults/main.yml:
- Added defaults for redis_syslog_facility and redis_logfile.
handlers/main.yml:
- Handler for restarting rsyslogd on conf change.