Fail tests on lint warnings

Both for the sake of clean code, and catching some more serious problems
like unscoped variables. Unfortunately this isn't the default.
Disable lint checks for Puppet 2.6 compat

This enables us to set mandatory params on classes. Which is desirable,
because sometimes there may be no sensible default - you want to blow up if
nothing has been provided from hiera.
Make jenkins_hostname param mandatory

The nginx::vhost::proxy resource needs this value and there's no way that we
can derive a default for it. Or rather, we shouldn't here, it should be in
hiera. So fail() the catalog if one isn't provided.