Use addCleanup instead of tearDown in test_sql_persistence

TestCase.tearDown() is not called if setUp raises exception,
but we have to close backend and release lock before executing
other tests. The easiest way to do it is to use addCleanup
instead of tearDown for freeing resources.

Change-Id: I885bb7d491f4b3ee06a351f5d22087e23804e19b
Ensure we register the new zookeeper backend as an entrypoint

Change-Id: I670dc73ac44dfb94320393a89ea2c61c575de1cd
Test fetching backends via entry points

Default way of getting backend is expected to be via directly or
indirectly using taskflow.persistence.backend.fetch(), so we should
test that all our backends can be fetched correctly.

Change-Id: I5ce41510c86598d449ee15da8d1279d3d35c9524
Merge "Ensure we register the new zookeeper backend as an entrypoint"
Merge "Test fetching backends via entry points"
Merge "Use addCleanup instead of tearDown in test_sql_persistence"