library/jenkins_node: Allow to work on centos7 by adding xml parser

library/jenkins_node: Allow to work on centos7 by adding xml parser

xmltodict is exactly what I needed, but it's too hard to dig up
for centos7 (and overkill anyway).  Supply an implementation that
allows jenkins XML parsing and use it instead.

Also, use python3 on centos7, and tweak the packages a bit in
examples/builder.yml

Signed-off-by: Dan Mick <dmick@redhat.com>