Add systemd-containers functional tests
Add systemd-containers functional tests

Add functional tests for the Ubuntu systemd-container elements
introduced with I45651de2aa1b19bdeee301094f0bdffdd0a3b45c

These test the Xenial and Bionic guest images build on Xenial and
Bionic hosts.  I'm not sure it's worth the gate resources to also do
this on CentOS/Trusty at this point, so they're left out.

Change-Id: Ie9da9e22f5421999ac96750624f7d53bb0aa6160

switch-ch/diskimage-builder,switch-ch/diskimage-builder,openstack/diskimage-builder,openstack/diskimage-builder