Given there is almost no difference between the releases, we can
use the same vars file and simple conditionals. The package
'software-properties-common' is available for Trusty & Xenial so
we just use that and remove the unnecessary extra package.
We also now add the correct UCA repositories for Trusty and Bionic
so that we get the latest version of libvirt.
Finally, we simplify the conditional for the iptables binary to
make it far easier to read.
Change-Id: Id4b3711a4d7a0ccc13db956d41017ac01c97825f
241837fc9f