Retry to connect database when DB2 or mongodb is restarted

The patch https://review.openstack.org/#/c/122387 works fine
with operations with get, record and update functions.
But exception would still occured with the operation of
db.collection.find() function.

This patch can give some benefit to tolerate DB restart
with find() function.
This patch also removes "test_mongo_find" test case since
it doesn't raise AutoReconnect exception at all.

Change-Id: Ia0474726960ce2b4b611fda0a1c304bb8ad96922
Closes-Bug: #1389985
Merge "Retry to connect database when DB2 or mongodb is restarted"