Avoid jinja2 templating in when warnings

Exerpt from log:
```
[WARNING]: when statements should not include jinja2
templating delimiters such as {{ }} or {% %}. Found:
{{ neutron_plugin_type == 'ml2.ovs.dvr' }}
```

Change-Id: Ie628be544a2f33212987c2d3b64f422fb5deb382

playbooks/common-tasks/dynamic-grouping.yml

@@ -19,6 +19,6 @@
     name: "{{ item }}"
   with_items: "{{ groups[src_group] }}"
   when:
-    - "{{ groups[src_group] is defined }}"
+    - "src_group in groups"
   tags:
     - common-grouping