Ansible won't parse a variable not defined even if the directive is to do nothing if not defined. Ok.

Use the "user" module.

Author: Christopher M. Neill

ansible/redis-slave.yml

@@ -9,3 +9,4 @@
   - { role: container_kill_start,
       container_run_args: "redis-server --slaveof {{ redis_host_address }} 6379",
       tags: "deploy" }
+  - { role: datadog }

ansible/roles/datadog/tasks/main.yml

@@ -32,8 +32,10 @@
 - name: add docker to dd-agent group
   tags: ['datadog']
   sudo: yes
-  when: not no_dd_monitoring
-  shell: usermod -a -G docker dd-agent
+  when: no_dd_monitoring != false
+  user:
+    name=dd-agent
+    groups=docker
 
 - name: create dd configdir
   tags: ['datadog']
@@ -64,7 +66,7 @@
 - name: install docker integration
   tags: ['datadog']
   sudo: yes
-  when: not no_dd_monitoring
+  when: no_dd_monitoring != false
   template:
     src=docker_daemon.yaml.j2
     dest=/etc/dd-agent/conf.d/docker_daemon.yaml