add dstat to see top process info

pidstat data isn't exceptionally useful, it's lots of lines, and
seems to be missing some of the most critical one.

dstat is kind of like sysstat, except the formatting is much better.
It also supports advanced features like the top CPU using process
at every interval.

put this behind the sysstat variable, as we'll probably want to
replace sysstat & pidstat with this if it works

Change-Id: I48dc22a0a7e63fe3abb527646cc70525998a7d85
Merge "add dstat to see top process info"
Merge commit 'refs/changes/99/70999/4' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch