Only Build Ubuntu 16.04 Xenial Xerus VMs
Testing revealed that we cannot build Ubuntu 16.04 VMs and still build
VMs with older releases. Since Xenial Xerus is the older LTS at this
point and Trusty Tahr (Ubuntu 14.04) will go out of LTS in April 2019,
we will focuse on Xenial.
I should also note that we cannot build Ubuntu 18.04 Bionic Beaver VMs
with vmbuilder, either. The move to netplan for networking
configuration will require big changes to the vmbuilder templates, and
no one in the vmbuilder community seems to be taking that on. This
could be the end of the road for our use of vmbuilder.
Signed-off-by: Jason Stephenson <jason@sigio.com>