Added a concept of 'valid' roles.

Review: https://reviews.apache.org/r/11206/
Added a json http endpoint for getting information about roles.

Review: https://reviews.apache.org/r/11210/
Added weights to DRFSorter.

Review: https://reviews.apache.org/r/11212/
Added tests for weighted DRF.

Review: https://reviews.apache.org/r/11214/
Updated docs to reflect changes to the allocator.

Review: https://reviews.apache.org/r/11529/