Fix issue with decorator 5.0.0

looks like support of python2 is dropped.

Change-Id: Ic98d85add73febffbd0f6fdc8d7fe7cc19003e53
Related-Prod: https://mirantis.jira.com/browse/PRODX-3632
diff --git a/src/com/mirantis/mk/Openstack.groovy b/src/com/mirantis/mk/Openstack.groovy
index 0785bbe..8f5271b 100644
--- a/src/com/mirantis/mk/Openstack.groovy
+++ b/src/com/mirantis/mk/Openstack.groovy
@@ -66,6 +66,7 @@
         'python-heatclient==1.18.0',
         'docutils==0.16',
         'pyrsistent<0.17.1',
+        'decorator<5.0.0',
     ]
 
     if (version == 'kilo') {