Use the default flavor name when flavor parameter is not defined

This patch ensures that the default flavor is used when the flavor
parameter (Overcloud{{role.name}}Flavor) is not explicitly defined
in deployment templates and plan, to emulate the behavior in tht
to pick up the defalt flavor to deploy baremetal instances.

Closes-Bug: #1901472
Change-Id: I6970a7fc294305d4fe6ad43b9c2e29090a1ecb37

lookup_plugins/roles_info.py

@@ -63,7 +63,7 @@ class LookupModule(LookupBase):
             return {
                 'name': role['name'],
                 'count': role.get('CountDefault', 0),
-                'flavor': None
+                'flavor': role.get('FlavorDefault', 'baremetal')
             }
 
         roles = list(map(default_role_data, roles_data))