if key name already exists on tenant, check if public_key is offered that it matches the existing key

cloud/nova_keypair

@@ -115,6 +115,9 @@ def main():
     if module.params['state'] == 'present':
         for key in nova.keypairs.list():
             if key.name == module.params['name']:
+                if module.params['public_key'] and (module.params['public_key'] != key.public_key ):
+                    module.fail_json(msg = "name {} present but key hash not the same as offered.  Delete key first.".format(key['name']))
+                else:
                     module.exit_json(changed = False, result = "Key present")            
         try:
             key = nova.keypairs.create(module.params['name'], module.params['public_key'])