Allow the deployment group to be optional

This small change prevents the deployment group from being mandatory.

Change-Id: I5693fbad0d5677ae341121619b6ec2e8738cffb7

ansible/roles/prechecks/tasks/main.yml

 ---
 - include_tasks: port_checks.yml
   when:
-    - inventory_hostname not in groups['deployment']
+    - inventory_hostname not in groups['deployment']|default([])
 
 - include_tasks: service_checks.yml