There are issues with pip packages and a python3 only Xenial systems.
This is occuring after Ie609de51cc5fcde701296c9474e315981d9778a2.
We believe the issue is with VIRTUAL_ENV being set within the chroot
and messing up pip installs
(Icdb769541eee9793f261b4b8ec563be76ee13fe2) but a full solution is not
yet clear.
For now, set Xenial to ensure we use python2. Install the package for
the ubuntu element (75-debian-minimal-baseinstall will install python2
for the minimal elements).
Change-Id: Id403919b0af93b375a900186c01a0d3a3bdfafea
