metal provider name attribute and fog key pair name resolution

Added new attribute so ssh keys can be loaded from a different data bag ['chef-server-cluster']['metal-provisioner-key-name'].  I saw the chef-vault comment, but figured this was a good first step.

fog_key_pair resource sets the name based on the node['chef-server-cluster']['aws']['machine_options']['bootstrap_options']['key_name'] attribute.

These changes allowed my mvp repo to succeed. https://github.com/patrick-wright/csc-repo