Replace attribute method call with explict precedence setting.

Silences warnings like:

WARN: Setting attributes without specifying a precedence is deprecated and will be
removed in Chef 11.0. To set attributes at normal precedence, change code like:
`node["key"] = "value"` # Not this
to:
`node.set["key"] = "value"` # This
Document the user and group instance attributes
Add name to metadata.rb, fixes FC045