Implement Chef Vault support for knife bootstrap

Solving issue https://github.com/opscode/chef/issues/1887

Tested on Chef 12 and Chef 11 ... :+1: 

I might need to push back this to Chef 11 since client needs to use it ASAP... 
register_client is not only useful for the chef-vault use case, it needs to be broken out as something that is done on every bootstrap and needs to default to on for Chef-12.