Add sqlalchemy.exc.OperationalError to the retry decorator

* Currently Mistral retries a DB transaction only in case of a DB
  deadlock (often happens on MySql) and a connection error. Both
  make sense to retry because the issue may be temporary. This
  patch also adds sqlalchemy.exc.OperationalError to the list of
  retriable exceptions since part of the errors wrapped into this
  exception may also be temporary, such as "Too many connections"
  error thrown by MySql. Some errors may not make sense to retry
  though (like SQL error) but this shouldn't be a problem because
  most of them will happen during development/testing time and
  will be fixed before going in production and even if it happens
  in a real production the worst thing that will happen is retrying
  a DB transaction up to the maximum configured number of attempts,
  currently hardcoded 50 times.

Change-Id: Ie2fe988cdb8e4ca88c3e51f510d87320d3fca9a6
Closes-Bug: #1796242

mistral/db/utils.py

@@ -12,10 +12,15 @@
 #    See the License for the specific language governing permissions and
 #    limitations under the License.
 
+from __future__ import absolute_import
+
 import functools
 
+from sqlalchemy import exc as sqla_exc
+
 from oslo_db import exception as db_exc
 from oslo_log import log as logging
+
 import tenacity
 
 from mistral import context
@@ -25,7 +30,11 @@ from mistral.services import security
 
 LOG = logging.getLogger(__name__)
 
-_RETRY_ERRORS = (db_exc.DBDeadlock, db_exc.DBConnectionError)
+_RETRY_ERRORS = (
+    db_exc.DBDeadlock,
+    db_exc.DBConnectionError,
+    sqla_exc.OperationalError
+)
 
 
 def _with_auth_context(auth_ctx, func, *args, **kw):