Use re2 compatible regexes in branch matchers

Python regexes are deprecated in Zuul, so update the use of regexes
in branch matchers with re2-compatible syntax.
The commit also removes matches for branches that are EOL - all
branches until stable/rocky (including).

Change-Id: I43c696ef2029cbf351f902b293df05f296dbb2a2

.zuul.yaml

@@ -342,7 +342,9 @@
 
 - job:
     name: grenade-dsvm-neutron-forward
-    branches: ^(?!(stable/(ocata|pike|queens|rocky|stein|train))).*$
+    branches:
+      regex: ^stable/(stein|train)$
+      negate: true
     parent: grenade-forward
 
 - job: