Added an overload of Resources::contains for Resource object.

Review: https://reviews.apache.org/r/29134
Added a method in Resources to return all persistent disks.

Review: https://reviews.apache.org/r/29177
Disallowed empty DiskInfo.

Review: https://reviews.apache.org/r/29178
Renamed totalResources in master to offeredResources.

Review: https://reviews.apache.org/r/29179