Doc - Detailing  objects' attributes available for policy.json

Adding to documentation ("Configuring Keystone" part) details about
target objects attributes, that are available to customize access
policies in the policy.json.

DocImpact
Change-Id: I3799942230062f4feaa9faf59097bf0ef8bc139b
Merge "Doc - Detailing  objects' attributes available for policy.json"