Enable ddtrace on staging

This is already the setup, which was applied from
https://github.com/dimagi/commcare-cloud/compare/ap/test-ddtrace-run .
This commit is just a cleaner version of that

environments/staging/app-processes.yml

@@ -1,6 +1,10 @@
 formplayer_command_args: ''
 # Use the following to enable datadog tracing
 # formplayer_command_args: '-javaagent:/home/cchq/dd-java-agent.jar -Dsrc.main.java.org.javarosa.enableOpenTracing=true'
+
+django_command_prefix: '{{ virtualenv_home }}/bin/ddtrace-run '
+datadog_pythonagent: True
+
 management_commands:
   celery_a1:
     handle_survey_actions: