Set bash as shell when executing mariadb recovery task

Added 'executable' argument to the shell action in the
'Comparing seqno value' task in the cluster recovery playbook.

Change-Id: I3e96a4a76b44ffb558b9a41cde16e66a8d0fab1a
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