Multi subnets support

StarCluster in VPC mode only supported a single VPC.
We want many to allow us to bid in the cheapest zone.
This code checks if the subnet has an route table  **explicitly associated**  to it and if that route table has a route for 0.0.0.0

This is wrong since a subnet will use the 'main' route table by default and that implicit association cannot be matched by `get_route_tables()` filters. Thus, even if the subnet can reach 0.0.0.0/0 via the default route table, the check would fail.

We decided to remove the check since it was not easy to fix properly and because it works fine in our environment.  Upstream may disagree.