Workaround for ansible jinja2 template bug

The jinja2 `variable_start_string` appears to be broken in ansible 1.6.6+ (see https://github.com/ansible/ansible/issues/4638)

This is the same workaround as deployed in https://github.com/basho/ansible-riak/pull/7

(FWIW, I still wasn't able to get this playbook to build, as it exited with

```
TASK: [basho.riakcs | configure /etc/riak-cs/app.config] **********************
fatal: [localhost] => {'msg': "AnsibleUndefinedVariable: One or more undefined variables: 'dict object' has no attribute 'stanchion'", 'failed': True}
fatal: [localhost] => {'msg': "AnsibleUndefinedVariable: One or more undefined variables: 'dict object' has no attribute 'stanchion'", 'failed': True}

FATAL: all hosts have already failed -- aborting
```

Full output: https://gist.github.com/swenson/5064772d3ce633d6024b)