POC: replace our external log aggregation feature with rsyslog

- this change adds rsyslog (https://github.com/rsyslog/rsyslog) as
  a new service that runs on every AWX node (managed by supervisord)
  in particular, this feature requires a recent version (v8.38+) of
  rsyslog that supports the omhttp module
  (https://github.com/rsyslog/rsyslog-doc/pull/750)
- the "external_logger" handler in AWX is now a SysLogHandler that ships
  logs to the local UDP port where rsyslog is configured to listen (by
  default, 51414)
- every time a LOG_AGGREGATOR_* setting is changed, every AWX node
  reconfigures and restarts its local instance of rsyslog so that its
  fowarding settings match what has been configured in AWX
- unlike the prior implementation, if the external logging aggregator
  (splunk/logstash) goes temporarily offline, rsyslog will retain the
  messages and ship them when the log aggregator is back online
- 4xx or 5xx level errors are recorded at /var/log/tower/external.err

awx/settings/defaults.py

@@ -1011,8 +1011,9 @@ LOGGING = {
             'formatter': 'simple',
         },
         'external_logger': {
-            'class': 'awx.main.utils.handlers.AWXProxyHandler',
+            'class': 'logging.handlers.SysLogHandler',
             'formatter': 'json',
+            'address': ('localhost', 51414),
             'filters': ['external_log_enabled', 'dynamic_level_filter'],
         },
         'tower_warnings': {