Add support for multiple cloud providers

This serves to fix the merge conflicts in https://github.com/jtimberman/ossec-cookbook/pull/13

Info from PR #13:

When running OSSEC on an infrastructure that spans multiple cloud providers, using node.ipaddress is not effective as the hosted machine is frequently NAT'd. This results in the clients trying to use a private address to communicate with the OSSEC server on a different network.

In order to work around this problem, I'd like to introduce an attribute which will instruct the recipes to consult the node's cloud attribute in order to obtain public addresses. This allows the infrastructure to span multiple cloud providers. Since most providers have reachability on both public and private addresses, there should be no caveats other than a change in the network route.

Default is disabled for backward compatibility.