Update test_helpers.sh

stemcell_os_name is not required in publish_vagrant_box as we only
publish trusty boxes

Signed-off-by: Kris Hicks <krishicks@gmail.com>
wip

Signed-off-by: Andrea Law <alaw@pivotallabs.com>
Signed-off-by: Maria Shaldibina <mshaldibina@pivotal.io>