libraries/helpers: minor fixes
libraries/helpers: minor fixes

* Only define constants if they're not defined.
* Describe what 'bootstrap' is about.
* Use Chef::Search::Query since we're not in the recipe DSL.

chef-cookbooks/chef-server-cluster,dcallao/chef-server-cluster,mbrukman/chef-server-cluster,chef-cookbooks/chef-server-cluster,mbrukman/chef-server-cluster,dcallao/chef-server-cluster