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/external_logging.py

@@ -32,7 +32,7 @@ def construct_rsyslog_conf_template(settings=settings):
         '$IncludeConfig /var/lib/awx/rsyslog/conf.d/*.conf',
         '$ModLoad imuxsock',
         'input(type="imuxsock" Socket="' + settings.LOGGING['handlers']['external_logger']['address'] + '" unlink="on")',
-        'template(name="awx" type="string" string="%msg%")',
+        'template(name="awx" type="string" string="%rawmsg-after-pri%")',
     ])
     if protocol.startswith('http'):
         # https://github.com/rsyslog/rsyslog-doc/blob/master/source/configuration/modules/omhttp.rst