fix a bug that broke bot avatars for Slack notifications

when a *color* is specified for a Slack notification template, we use
the *web* not the RTM API; when you use a bot with the web API, you have
to specify the `as_user=True` argument to have the message use the bot's
name and avatar

see: https://github.com/ansible/tower/issues/2883
see: https://github.com/ansible/awx/issues/2187

awx/main/notifications/slack_backend.py

@@ -69,6 +69,7 @@ class SlackBackend(AWXBaseEmailBackend):
                         r = r[1:]
                     ret = connection.api_call("chat.postMessage",
                                               channel=r,
+                                              as_user=True,
                                               attachments=[{
                                                   "color": self.color,
                                                   "text": m.subject