Funtional Tests should call glance.db.get_api

The test_registry , test_simple and test_sqlalchemy tests import the
modules directly instead of calling get_api of db/__init__.py
With the introduction of db_pool, they should use the get_api method.

Also adds a tear down to the ThreadPoolWrapper tests to fix jenkins
issues related to the CONF using tpool set to True in the tests.

Fixes Bug #1223167
Change-Id: I66e9f5fba449107ad92e895f6a9e550e0bcddf1c