work on pip robustness

Trying to diagnose why pip is failing on dashboards so much.
This change puts pip downloaded tarballs in same directory that
the cmake downloaded tarballs do.
It also make pip logging more verbose so we have a better
chance to see what went wrong.
make dashboards do clean pip builds
Merge remote-tracking branch 'devel/improve-pip-robustness'

* devel/improve-pip-robustness:
  make dashboards do clean pip builds
  work on pip robustness