Allow setting default execution group pod spec

This will allow us to control the default container group created via settings, meaning
we could set this in the operator and the default container group would get created with it applied.

We need this for https://github.com/ansible/awx-operator/issues/242

Deepmerge the default podspec and the override

With out this, providing the `spec` for the podspec would override everything
contained, which ends up including the container used, which is not desired

Also, use the same deepmerge function def, as the code seems to be copypasted from
the utils

awx/main/managers.py

@@ -179,7 +179,9 @@ class InstanceManager(models.Manager):
             else:
                 registered = self.register(ip_address=pod_ip, uuid=settings.SYSTEM_UUID)
             RegisterQueue(settings.DEFAULT_CONTROL_PLANE_QUEUE_NAME, 100, 0, [], is_container_group=False).register()
-            RegisterQueue(settings.DEFAULT_EXECUTION_QUEUE_NAME, 100, 0, [], is_container_group=True).register()
+            RegisterQueue(
+                settings.DEFAULT_EXECUTION_QUEUE_NAME, 100, 0, [], is_container_group=True, pod_spec_override=settings.DEFAULT_EXECUTION_QUEUE_POD_SPEC_OVERRIDE
+            ).register()
             return registered
         else:
             return (False, self.me())