Remove default empty dict in case of undefined attribute

Previous line always evaluated to false/empty dict which
caused to always try and use groups.tester even if groups.undercloud
was defined.

Change-Id: I43bd9719d866894a0a829704683a5cf0fac8dfb3

main.yml

@@ -4,7 +4,7 @@
   tasks:
     - name: Add chosen node to tester group
       add_host:
-          name: "{{ install.get('tester', {}).get('node')|default(groups.get('undercloud', {})|first, true)|default(groups.tester|first, true) }}"
+          name: "{{ install.get('tester', {}).get('node', {})|default(groups.undercloud|first, true)|default(groups.tester|first) }}"
           groups: "tester"
     - include_role:
           name: inventory-update