fix(unit_tests/test_config): clear env variables

those test are building heavily on `SCT_*` env variables
and seems like other unit tests might be leaving behind
some of those variables in a way it's breaks
`ConfigurationTests.test_02_verify_config` in the following way:

```
>           raise ValueError("not all multi region values are equal: \n\t{}".format(region_count))
E           ValueError: not all multi region values are equal:
E           	{'region_name': 1, 'n_db_nodes': 2, 'ami_id_db_scylla': 1, 'ami_id_loader': 1}
```

unit_tests/test_config.py

@@ -36,7 +36,7 @@ def setUpClass(cls):
         logging.getLogger('botocore').setLevel(logging.CRITICAL)
         logging.getLogger('boto3').setLevel(logging.CRITICAL)
         logging.getLogger('anyconfig').setLevel(logging.ERROR)
-
+        cls.clear_sct_env_variables()
         cls.setup_default_env()
 
         os.environ['SCT_CLUSTER_BACKEND'] = 'aws'