Fix scenario import and add test UUIDs

The scenario manager module referenced tempest.scenario.manager without importing it, causing AttributeError when check-uuid scanned test classes. An unused tempest.test import was also present.

This change adds the missing import, removes the unused import, and creates a uuidgen testenv. Running check-uuid after the fix generated idempotent_id decorators for 30 test methods across 8 API test modules.

Check-uuid command comes from tempest package, available in requirements.txt, hence added to tox.ini file to install tempest.