Implement "symlinks" deploy attribute consistant with documentation

According to the [OpsWorks documentation](http://docs.aws.amazon.com/opsworks/latest/userguide/attributes-json-deploy.html#attributes-json-deploy-app-symlinks) there should be a "symlinks" attribute which gets passed to the chef deploy resource.

It can even be seen when running `opsworks-agent-cli get_json`, so it seems as though it should do something, however the deploy cookbook currently ignores the attribute.  This PR should fix it.

I have added a default consistent with the definition at:
<https://github.com/opscode/chef/blob/469bed/lib/chef/resource/deploy.rb#L70>