[COOK-4277] Attributize status_log attribute

The default location of the status_log LWRP attribute causes the service
start to fail under CentOS w/SeLinux enabled. This can be mitigated by
changing the status_log setting to use /var/log/openvpn, which has the
correct SEL context. In order to to that, the attribute needs to be
changeable using node attributes.

- Add openvpn/status_log attribute
- Change LWRP call in default recipe to pass status_log attribute
- Move LWRP attribute default to attributes file like other configurable
  attributes
- Update metdata with new attribute
- Update README with new attribute

See: https://tickets.opscode.com/browse/COOK-4277