Failed to update apt cache when executing lampstack workload

Description
===========
When i executed `lampstack` workload, sometimes the task 'update apt cache'
was broken.

Steps to reproduce
==================
* Clone the latest source
git clone https://github.com/openstack/interop-workloads.git
* Create my cloud's vars file
* Execute ansible-playbook
ansible-playbook -e "action=apply env=inspur password=interop" site.yml

Expected result
===============
SUCCESS

Actual result
=============
Failure

Environment
===========
1. interop-workloads version
# git log
commit a25de91b6e77487060d3035b4202a48e0a5b2b2f
Merge: 3e4d4e8 2c89e57
Author: Jenkins <email address hidden>
Date: Wed Mar 22 14:55:01 2017 +0000

    Merge "Kubernetes workload"

2. Which workload did you use?
lampstack

Logs
===========
TASK [update apt cache] ********************************************************
fatal: [172.23.48.51]: FAILED! => {"changed": false, "failed": true, "msg": "Failed to lock apt for exclusive operation"}
fatal: [172.23.48.49]: FAILED! => {"changed": false, "failed": true, "msg": "Failed to lock apt for exclusive operation"}
fatal: [172.23.48.50]: FAILED! => {"changed": false, "failed": true, "msg": "Failed to lock apt for exclusive operation"}