There was an issue in pip that prevented correctly caching locally
built wheels [1]. This has been fixed in recent pip versions so
upgrade pip in both images so image caching works correctly. This is
needed to unbreak nodepool images that fail to install the locally
built netifaces package.
[1] https://github.com/pypa/pip/issues/6852
Change-Id: Ibbe12bcc53253a80d0bafa3d09a20c49a3a2b784
e35fcde591