Add python3 functional test job

This was created and added in the keystone repo so let's use it here as
well.

Change-Id: I0686d52730b0d773730baea84710bda213c402b3
diff --git a/.zuul.yaml b/.zuul.yaml
index 57dcc3b..6881f2f 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -5,6 +5,7 @@
     check:
       jobs:
         - keystone-dsvm-functional
+        - keystone-dsvm-py3-functional
         - keystone-dsvm-functional-federation:
             voting: false
         - keystone-dsvm-py35-functional-federation:
@@ -12,3 +13,4 @@
     gate:
       jobs:
         - keystone-dsvm-functional
+        - keystone-dsvm-py3-functional