Fix handling of switch_type variable in Ansible 2.5

In Ansible, playbook group variables take precedence over inventory
group variables. Typically in a kayobe-config we might specify the
switch_type variable for a group containing switches using inventory
group_vars. This seemed to work in Ansible 2.3, but since Ansible 2.4
the default value of None in kayobe's playbook group_vars takes
precedence. This behaviour is as documented, not sure why it worked
previously.

This change removes the default value of None set in the playbook
group_vars.

Change-Id: I042ad8d115cdc0dd57a116e1be9d397ef4d216ae
Story: 2002903
Task: 22870

ansible/group_vars/switches/type

@@ -1,7 +0,0 @@
----
-###############################################################################
-# Switch type configuration.
-
-# This configures the type of switch, which is used to determine how the switch
-# will be configured.
-switch_type:

ansible/physical-network.yml

@@ -19,6 +19,12 @@
     # Set this variable to True in order to display the candidate switch
     # configuration and exit without applying it.
     physical_network_display: False
+    # List of supported values for the 'switch_type' variable.
+    supported_switch_types:
+      - dell
+      - dell-powerconnect
+      - junos
+      - mellanox
   tasks:
     - name: Fail if both interface name and description limits are specified
       fail:
@@ -29,6 +35,14 @@
         - physical_network_interface_limit != ''
         - physical_network_interface_description_limit != ''
 
+    - name: Fail if the switch type is not defined
+      fail:
+        msg: >
+          The switch type is not defined or is unrecognised. Configure the type
+          of each host in the 'switches' group via the 'switch_type' variable.
+      when: switch_type is not defined or
+            switch_type not in supported_switch_types
+
     - name: Group hosts by their switch type
       group_by:
         key: "switches_of_type_{{ switch_type }}"