Ddr::Collection returns its own id as its admin_policy_id; DDR-1613.

This MR "hard-codes" the admin_policy_id method of a Ddr::Collection to return its own id. This is a change in behavior from the current DDR code base, which utilizes an after_create callback to set the admin policy of a Collection to itself. Getting that callback to work in the DDRevo code base (in ddr-admin) was proving difficult so I opted to propose this as an alternative. While it is technically different behavior, I do not believe that different is significant, given how the DDR actually functions currently. If there are objections to this approach or if it proves problematic later, we can revisit the callback approach.