Add rsyslog and made vm.overcommit_memory parameterizable

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.