(maint) Update default hostname for forge

The forge acceptance test host was renamed last night to
'vulcan.delivery.puppetlabs.net', but the test harness is still
defaulting to the deprecated hostname. This commit updates the test
harness to use the new name of the forge acceptance host.
Merge pull request #388 from adrienthebo/update_forge_acceptance_host

(maint) Update default hostname for forge