elasticsearch/Dockerfile.template: Log to syslog instead of a local file

Use Log4j's SyslogAppender [1] to log to a local Syslog.  The Syslog
appender doesn't use /dev/log [2], so we also need to Syslog to listen
on the UDP socket.  Relevant threads from the Elasticsearch mailing
list [3,4].

[1]: http://logging.apache.org/log4j/2.x/manual/appenders.html#SyslogAppender
[2]: https://issues.apache.org/bugzilla/show_bug.cgi?id=44839
[3]: https://groups.google.com/forum/#!topic/elasticsearch/zeGWxzSQ4N4/discussion
     Subject: Using the syslog appender in elasticsearch
     From: Darron Froese
     Date: 2011-12-28
[4]: https://groups.google.com/forum/#!topic/elasticsearch/-ydujtOC5Fw/discussion
     Subject: use elasticsearch with syslog
     From: Olivier Morel
     Date: 2013-04-30