The pip-and-virtualenv element provides pip inside the created images.
When this pip is used inside the chroot of the image it, by default, creates
a cache directory with the http packages and the resulting wheels.
In the case of the octavia ubuntu-minimal image this cache is using ~50MB of
cache that is not needed after the image finished building.
This package creates a finalise.d task that removes the cache from the
default location.
Change-Id: I4715437b068d04993ef755bd1e27963db1d22417
