[COOK-1344] Add "delete password on create user" feature

http://tickets.opscode.com/browse/COOK-1344

I found a need to delete user passwords when creating them, so they can change it conveniently.

If `delete_password_on_create` is true, when user is created, his
password is deleted (using passwd -d USER). Then, when user logs into
the system using his ssh key, he can create a new password without
specifying the "old" one (which does not make sense when creating a user
via Chef).