Enable pidstat by default

pidstat comes from the sysstat package and will allow us to get some
per-process CPU/memory information for devstack runs.  pidstat was added
to devstack in the following change:

    https://review.openstack.org/#/c/68702

Change-Id: Iee001c8fd01aba9c9c98c2b15361ffb8d0346813
Merge "Enable pidstat by default"