Fix Ubuntu installation notes on KB
The installation procedure here: https://kb.isc.org/docs/kea-build-on-ubuntu needs some corrections and updates.
it still mentions --with-tier2 (which was removed in 1.4.0)
the Radius installation section mentions FreeRADIUS client being installed from the upstream. Francis did some important (segfault preventing) fixes in his version. See https://jenkins.isc.org/job/Kea_doc/guide/kea-guide.html#hooks-radius. It may be ok to use the upstream if they accepted Francis' patches and released fixed version.
It doesn't cover NETCONF. This is pretty important, because sysrepo is pretty darn tricky to install and Ubuntu happens to be a least painful system to get it on.