Fixed devstack installation to handle datasource config

A recent commit changed the datasource configuration but
failed to update the devstack installation procedure
to reflect those changes. In particular the installation
should copy a sample datasource configuration into
/etc/congress and set the appropriate variable in the config file.

This change includes both fixes.

Change-Id: I1cd99f109c1eb27dfb63d90ea353dcac2f8edb7e