Update to chef-zero 3
Look up username instead of resource name

The username isn't always the same as the resource name, and we incorrectly
use the latter when determining if the user exists in #load_current_resource.

Fixes #1705.
Make exception for a user not existing more explicit
unpack the conditionals a little for readability
Changelog update for #1705
Use path_join to create local-mode-cache
Always give config_dir a value
CHANGELOG.md for 1854
Merge pull request #1854 from opscode/lcg/config_dir

Use path_join to create local-mode-cache
CHEF-4101: DeepMerge - support overwriting hash values with nil
add CHANGELOG.md for PR#719
Merge pull request #1851 from opscode/lcg/chef-4101

CHEF-4101: DeepMerge - support overwriting hash values with nil