[ussuri][goal] Drop python 2.7 support and testing

OpenStack is dropping the py2.7 support in ussuri cycle.

heat-tempest-plugin is ready with python 3 and ok to drop the
python 2.7 support.

Stable zuul jobs are modified to run with py3 as heat-tempest-plugin
cannot run py2 jobs on master gate.

Complete discussion & schedule can be found in
- http://lists.openstack.org/pipermail/openstack-discuss/2019-October/010142.html
- https://etherpad.openstack.org/p/drop-python2-support

Ussuri Communtiy-wide goal:
https://governance.openstack.org/tc/goals/selected/ussuri/drop-py27.html

Change-Id: Ida6919a70dc8711166a79eac574b0714382e8d00
diff --git a/.zuul.yaml b/.zuul.yaml
index d43343a..b1c8faf 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -1,6 +1,6 @@
 - job:
     name: heat-functional-convg-queens
-    parent: heat-functional-convg-mysql-lbaasv2
+    parent: heat-functional-convg-mysql-lbaasv2-py3
     nodeset: legacy-ubuntu-xenial
     override-checkout: stable/queens
     vars:
@@ -13,10 +13,11 @@
     nodeset: legacy-ubuntu-xenial
     vars:
       branch_override: stable/queens
+      use_python3: 1
 
 - job:
     name: heat-functional-convg-rocky
-    parent: heat-functional-convg-mysql-lbaasv2
+    parent: heat-functional-convg-mysql-lbaasv2-py3
     override-checkout: stable/rocky
     nodeset: legacy-ubuntu-xenial
     vars:
@@ -29,10 +30,11 @@
     nodeset: legacy-ubuntu-xenial
     vars:
       branch_override: stable/rocky
+      use_python3: 1
 
 - job:
     name: heat-functional-convg-stein
-    parent: heat-functional-convg-mysql-lbaasv2
+    parent: heat-functional-convg-mysql-lbaasv2-py3
     override-checkout: stable/stein
     nodeset: legacy-ubuntu-xenial
     vars:
@@ -45,10 +47,11 @@
     nodeset: legacy-ubuntu-xenial
     vars:
       branch_override: stable/stein
+      use_python3: 1
 
 - job:
     name: heat-functional-convg-train
-    parent: heat-functional-convg-mysql-lbaasv2
+    parent: heat-functional-convg-mysql-lbaasv2-py3
     override-checkout: stable/train
     nodeset: legacy-ubuntu-xenial
     vars:
@@ -61,6 +64,7 @@
     nodeset: legacy-ubuntu-xenial
     vars:
       branch_override: stable/train
+      use_python3: 1
 
 - project:
     templates: