Use /bin/bash as executable in a task of the mariadb recovery play

Change-Id: I5e5aaab4fd1ef39de1b1e6fcf5b87ba8b41ba7eb
Closes-bug: 1729603

ansible/roles/mariadb/tasks/recover_cluster.yml

@@ -42,6 +42,8 @@
 
   - name: Comparing seqno value on all mariadb hosts
     shell: "if [[ {{ hostvars[inventory_hostname]['seqno'] }} -lt {{ hostvars[item]['seqno'] }} ]]; then echo {{ hostvars[item]['seqno'] }}; fi"
+    args:
+      executable: /bin/bash
     with_items: "{{ groups['mariadb'] }}"
     changed_when: false
     register: seqno_compare