Use Bionic node for stable stein or train jobs

We have migrated our testing from Xenial to Bionic
since stein. Xenial node is not supported on stein onwards.

Any job running on stein onwards should use the bionic nodeset.

Change-Id: If8a018465f6ed3ec5c35c0934370c1107f4b9969
diff --git a/.zuul.yaml b/.zuul.yaml
index b1c8faf..2567325 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -36,7 +36,6 @@
     name: heat-functional-convg-stein
     parent: heat-functional-convg-mysql-lbaasv2-py3
     override-checkout: stable/stein
-    nodeset: legacy-ubuntu-xenial
     vars:
       branch_override: stable/stein
 
@@ -44,7 +43,6 @@
     name: heat-functional-orig-stein
     parent: heat-functional-orig-mysql-lbaasv2
     override-checkout: stable/stein
-    nodeset: legacy-ubuntu-xenial
     vars:
       branch_override: stable/stein
       use_python3: 1
@@ -53,7 +51,6 @@
     name: heat-functional-convg-train
     parent: heat-functional-convg-mysql-lbaasv2-py3
     override-checkout: stable/train
-    nodeset: legacy-ubuntu-xenial
     vars:
       branch_override: stable/train
 
@@ -61,7 +58,6 @@
     name: heat-functional-orig-train
     parent: heat-functional-orig-mysql-lbaasv2
     override-checkout: stable/train
-    nodeset: legacy-ubuntu-xenial
     vars:
       branch_override: stable/train
       use_python3: 1