core: use default color by default (not white)
Add support for chef-zero provisioner.

This is my second crack at adding support for the chef-zero
provisioner. Since we do not want to deprecate chef-solo we should add
this as a completely separate entity for now.
Add tests for chef_binary_path return values.

This return value differs for the chef-zero provisioner. In this
particular case it inherits the chef-solo provisioner but uses the
chef-client binary.