Don't call provision.py directly

When testr.conf calls the sqlalchemy/provision.py script directly,
sys.path[0] is the directory that invoked the script.  So when it
tries to import openstack.common.db you end up with an import failure.

---
Traceback (most recent call last):
  File "./openstack/common/db/sqlalchemy/provision.py", line 27, in <module>
    from openstack.common.db import exception as exc
ImportError: No module named openstack.common.db
---

The solution is to run it with -m

Closes-Bug: #1303579
Change-Id: Ia7ff187df1685f0dd5ef32c3f6de474218874b04

.testr.conf

@@ -14,6 +14,6 @@ test_list_option=--list
 # https://testrepository.readthedocs.org/en/latest/MANUAL.html#remote-or-isolated-test-environments
 # for details.
 
-instance_provision=${PYTHON:-python} ./openstack/common/db/sqlalchemy/provision.py create $INSTANCE_COUNT
-instance_dispose=${PYTHON:-python} ./openstack/common/db/sqlalchemy/provision.py drop $INSTANCE_IDS
+instance_provision=${PYTHON:-python} -m openstack.common.db.sqlalchemy.provision create $INSTANCE_COUNT
+instance_dispose=${PYTHON:-python} -m openstack.common.db.sqlalchemy.provision drop $INSTANCE_IDS
 instance_execute=OS_TEST_DBAPI_CONNECTION=$INSTANCE_ID $COMMAND