rsyslogd: use %rawmsg-after-pri% instead of %msg%

after some prolonged RFC reading and tinkering w/ rsyslogd...

cpython's SysLogHandler doesn't emit RFC3164 formatted messages
in the format you'd expect; it's missing the ISO date, hostname, etc...
along with other header values; the handler implementation relies on you
to specify a syslog-like formatter (we've replaced all of this with our
own *custom* logstash-esque formatter that effectively outputs valid JSON
- without dates and other syslog header values prepended)

because of this unanticipated format, rsyslogd chokes when trying to
parse the message's parts;  AWX is emitting:

<priority>RAWJSON

...so the usage of `%msg%` isn't going to work for us, because rsyslog
tries to parse *all* of the possible headers (and yells, because it
can't find a date to parse):

see: https://www.rsyslog.com/files/temp/doc-indent/configuration/properties.html#message-properties

this is fine, because we don't *need* any of that message parsing
anyways; in the end, we're *just* interested in forwarding the raw
JSON/text content to the third party log handler

awx/main/utils/formatters.py

@@ -97,7 +97,7 @@ class LogstashFormatterBase(logging.Formatter):
 
     @classmethod
     def serialize(cls, message):
-        return ' ' + json.dumps(message, cls=DjangoJSONEncoder) + '\000'
+        return json.dumps(message, cls=DjangoJSONEncoder) + '\n'
 
 
 class LogstashFormatter(LogstashFormatterBase):