Fix the destroy of 'aws_security_group' for mysql-primary/replica

[ChrisBAshton]

After #1530, we're hitting an error applying the plan:

Error: Error applying plan:

2 errors occurred:
	* aws_security_group.mysql-replica (destroy): 1 error occurred:
	* aws_security_group.mysql-replica: Error deleting security group: DependencyViolation: resource sg-26a3915d has a dependent object
	status code: 400, request id: 481cb159-77ee-46ef-813a-e82a9b91f754

	* aws_security_group.mysql-primary (destroy): 1 error occurred:
	* aws_security_group.mysql-primary: Error deleting security group: DependencyViolation: resource sg-d1bc8eaa has a dependent object
	status code: 400, request id: c9eee46f-f760-460f-929e-07e600d4c700

Trying a fix outlined in hashicorp/terraform-provider-aws#1671 (comment)

Trello: https://trello.com/c/VWpD8SvY/55-remove-mysql-primary-and-mysql-standby-infrastructure-and-configuration

[ChrisBAshton]

Turns out this fix didn't work for us - moreover, the problem only seems to affect Integration, so concentrating on Staging and Production for the moment.