reset prereqs status on clean.sh

when running a clean, we should really reset the prereq status as
well, as this should start us back from zeroish.

Change-Id: I5fae151ab13bcf7fb82feb1e91eed19e0215dc59
remove sysstat & pidstat

dstat is far cleaner for getting results out of the environment,
and covers the bulk of our use cases for sysstat and pidstat with
a much better ui.

devstack is allowed to be opinionated, so become opinionated here.

Change-Id: I21ec96339dcd704098512fdafd896738f352962d
Merge "reset prereqs status on clean.sh"
Merge "remove sysstat & pidstat"
Merge branch 'master' of github.com:openstack-dev/devstack