add dstat to our list of statistics collectors

this will probably become the replacement for sysstat and pidstat
in the gate, but for now, just add it to the list, so we can start
getting data with it and make sure it's useful enough to drop the
others.

Change-Id: I9bb79cb0b5b0f9d98d03c31cddcda9a756aea54f
Merge "add dstat to our list of statistics collectors"