refactor: configure PIP cache directory

To remedy errors we are seeing with intermittent package installation
from PyPi, introduce a PIP cache directory to the project filer. This
should allow Jenkins to share cached packages across nodes.

Change-Id: I9f607da75ebc7f6dc9b3de0f21a4c87bbfe02405
Signed-off-by: Chris Kay <chris.kay@arm.com>
1 file changed