Temporarily disabled health check test.

Health check test is flaky and has been temporarily disabled until we
have found the root cause.
Updated Sorter to allow transforming allocated resources.

Review: https://reviews.apache.org/r/29083
Updated Allocator to allow transforming allocated resources.

Review: https://reviews.apache.org/r/29084
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
Adjusted the calculation of unused resources in _launchTasks by
considering persistent disk acquisition.

Review: https://reviews.apache.org/r/28720
Replaced a bad allocator CHECK with a TODO.