Improved robustness of devstack install script for snapshot

Previously, the devstack install script assumed the
directory congress/etc/snapshot existed.  If it didn't,
the install would fail.

This change checks if the directory exists and
proceeds accordingly.

Change-Id: Ia4dfae34f9946f32b5bf8b35afcfbfc935882972
Merge "Improved robustness of devstack install script for snapshot"