Added a Resources transformation for acquiring persistent disk.

Review: https://reviews.apache.org/r/29128
Fixed a compilation issue on gcc 4.4.7.
Removed an unnecessary helper in the sorter tests.

Review: https://reviews.apache.org/r/29088
Fixed the DRFSorter share calculation to not assume flattened resources.

Review: https://reviews.apache.org/r/29089