Fix #8666 for real

Commit 311ec543af7605bdfcd30d74ea5f77448609b995 ("If not specified, do not modify subnet/route_tables for ec2 VPCs") mostly fixed the problem, except that it left defaults for subnets and route_tables so that not specifying them still deleted them.
Merge pull request #8868 from mike-d-d/fix-route_tables-default

Fix #8666 for real