tox: Disable pip cache for coala venv

JIRA: VAL-10

The Jenkins verify jobs report success even when there are failures,
because it doens't find the coala bears to run.

Ported https://gerrit.akraino.org/r/#/c/607/ to fix the issue

Also fixed the tox issues introduced by 537 as they are casuing the
verify job to fail:
 - conformance/sonobuoy.yaml is whitelisted as per Miguel's request

Change-Id: I497121b8bbd08cf43e7d7b25470f9814ca1d9227
Signed-off-by: Cristina Pauna <cristina.pauna@enea.com>