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/ui/client/src/configuration/forms/system-form/sub-forms/system-logging.form.js

@@ -78,7 +78,7 @@
                  ngClick: 'vm.testLogging()',
                  label: i18n._('Test'),
                  class: 'btn-primary',
-                 ngDisabled: 'configuration_logging_template_form.$invalid'
+                 ngDisabled: 'configuration_logging_template_form.$pending'
              },
              cancel: {
                  ngClick: 'vm.formCancel()',