Fix for #174 allows setting $jenkins::port

When used with strict variables turned on, firewall.pp expects
$jenkins::port to be set. Overriding in hiera will not correct as it's
still an undefined variable in code. This fix allows port to be set and
provides a default in params.pp
Merge pull request #204 from eshamow/fix_undefined_jenkins_port

Fix for #174 allows setting $jenkins::port