Fix dscl user provider to be able to manage home and password at the same time.

Alternative fix for https://github.com/opscode/chef/pull/1928

Currently dscl user provider can't manage the home directory and the password for a user at the same time. This PR makes sure the password modification is done with fresh user data in order not to overwrite previous changes. 

/cc: @opscode/client-engineers, @jaymzh