metalsmith_instances: Handle an empty instance list

The dynamic concurrency change[1] causes an issue when the instance
list is empty because the thread pool is created with max_workers=0.

This change avoids this issue by exiting the provision function early
if the instances list is empty.

[1] https://review.opendev.org/#/c/714735/
Change-Id: I0d3587b575b0d32563ef24c7b1316c0b173a4268

tripleo_ansible/ansible_plugins/modules/metalsmith_instances.py

@@ -235,6 +235,9 @@ def _release_nodes(provisioner, node_ids):
 
 
 def provision(provisioner, instances, timeout, concurrency, clean_up, wait):
+    if not instances:
+        return False, []
+
     # first, ensure all instances are reserved
     reserve(provisioner, [i for i in instances if not i.get('name')], clean_up)