Add back support for chef-solo/vagrant, lost in v0.4.0

When encrypted data bag support was added in v0.4.0, chef-solo/vagrant would fail because the password was nil, rather than defaulting to the node attribute password ("change_me").  

In chef-solo, this causes the set_admin_passwd.py script to be called without the second argument (password), so the script fails, failing the chef run.