Fix syntax in rax_clb_nodes that breaks in Python3 (#4933) (#4936)

* Use syntax that works in both Python 2 and 3 when iterating through a
    dict that's going to be mutated during iteration
  * Fixes `dictionary changed size during iteration` error
  * Fixes #4932

(cherry picked from commit 9a928d5ffb)

Co-authored-by: Teddy Caddy <tcaddy@users.noreply.github.com>

plugins/modules/cloud/rackspace/rax_clb_nodes.py

@@ -252,7 +252,8 @@ def main():
                 'weight': weight,
             }
 
-            for name, value in mutable.items():
+            for name in list(mutable):
+                value = mutable[name]
                 if value is None or value == getattr(node, name):
                     mutable.pop(name)