Run bashate job on bionic nodeset

bashate job on d-g fail with error, as d-g
is moving towards deprecated phase let's keep
bashate job running on bionic.

Change-Id: Ibe644c9ed207bf2f76895319774d7376e08391e3

.zuul.yaml

@@ -25,6 +25,8 @@
 - project:
     check:
       jobs:
+        - openstack-tox-bashate:
+            nodeset: ubuntu-bionic
         - devstack:
             files:
               - ^roles/