Made changes to support chef server that is not managed by Cloudify without requiring manual registration of chef server URL and validation cert to Cloudify Manager

Fixed https://cloudifysource.atlassian.net/browse/RECIPES-7: When use a chef server that is not managed by Cloudify, currently it requires to manually register the chef server URL and validation cert to Cloudify Manager. This extra manual step is not quite convenient and it is easy to forget in many situations, e.g., frequent bootstrapping of clouds for testing/development purpose.

It will be better if the chef recipe could pick up the chef server URL and validation cert from the properties file when they are not available in the attribute store.