devstack (stack.sh) fails when extra config files are specified

Latest merge of https://review.openstack.org/#/c/71996/ exposes an issue
in stack.sh which did not surface before.

Please see the details of the issue in the bug description.

Closes bug: 1285884

Change-Id: Ie231c9835497c2a418a61d339dfd5df1aab9e3d7
Merge "devstack (stack.sh) fails when extra config files are specified"