tempest-plugin-sanity: respect upper-constraints

tempest-tox-plugin-sanity-check zuul gates are currently failing for
plugins that use psycopg2, since the release of psycopg2 2.8.4. Global
upper constraints have the version pinned to 2.8.3.

This change updates the sanity check to use the global constraints.

Change-Id: I8bacf2d23a005e43c79d93086559b6f8462fc1bf
1 file changed