Introduces {name}-tox-{toxenv}-trusty

A copy paste of '{name}-tox-{toxenv}' but bound to UbuntuTrusty. Will
let pywikibot run py3.4 jobs.

Documents each templates as to which python versions are provided by the
different Ubuntu distributions we are using.

Change-Id: Ic01a876b96e78f5c94938a2b46f1737ae8aee2a5