Dynamically change poll interval in the reaper.

If pid count <= 50 then use 100 ms (<= 0.5% core usage), if count
>= 500 use 1000 ms (<= 1% core usage at 500 pids), else interpolate.

Review: https://reviews.apache.org/r/25947
Adds The Factory to PoweredByMesos documentation.
Added docker container prefix to docker docs.

Review: https://reviews.apache.org/r/26052