Qbs: Make sure a fresh QbsBuildStep has a configuration

Task-number: QTCREATOR-9355

Change-Id: I36b77a95300fc1574c2e5bebb57722e6b3cb433a
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>