Fix chart-testing role

It must use bash to be able to utilize the
source function when activating the virtualenv

Change-Id: Idf07040b8756361aa84ab31c29a73aba47032af7
Signed-off-by: Vladimir Kozhukalov <kozhukalov@gmail.com>

roles/chart-testing/tasks/main.yaml

@@ -4,3 +4,4 @@
     ct lint {{ chart_testing_options }}
   args:
     chdir: "{{ zuul_work_dir }}"
+    executable: /bin/bash