Update marconi.conf-sample to support stevedore

The change Ib601418a7dbcad84c79b640b93bc5798ec4a62c4 added support for
stevedore but it didn't update the config sample file under etc/

This patch updates that file.

Change-Id: Ie225c929a12059f077993db9ced3d49981897c8f
Merge "Update marconi.conf-sample to support stevedore"