Support sid for user resource

[chris-rock]

Description

The current user resource is not returning the sid for users:

describe user("Administrator") do
  it { should exist }
  its('uid') { should_not eq nil }
end

InSpec and Platform Version

0.31.0

Possible Solutions

We should return the sid as part of the user object: https://github.com/chef/inspec/blob/master/lib/resources/user.rb#L457-L463

This PR is depending on the improvement #86