Fix _ensure_default_security_group logic

Fix _ensure_default_security_group logic

In a case when first attempt to fetch default security group
fails and attempt to add it fails too due to a concurrent insertion,
later attempt to fetch the same default sg may fail due to
REPEATABLE READ transaction isolation level.
For this case RetryRequest should be issued to restart the
whole transaction and be able to see default group.

The patch also removes 'while True' logic as it's unsafe

Closes-Bug: #1475938
Change-Id: I20f65d3eae9421429aced1f4586cb6988ab577ff