aws flake aws_subnet.*: timeout

[s-urbaniak]

Reported in #1054 (comment)

* aws_subnet.priv_subnet[3] (destroy): 1 error(s) occurred:
* aws_subnet.priv_subnet.3: Error deleting subnet: timeout while waiting for state to become 'destroyed' (last state: 'pending', timeout: 10m0s)

[s-urbaniak]

When deleting subnets, "DependencyViolation" error is a retry-able error [1] with a timeout of 10 minutes [2].

[1] https://github.com/hashicorp/terraform/blob/v0.9.9/builtin/providers/aws/resource_aws_subnet.go#L303-L306
[2] https://github.com/hashicorp/terraform/blob/v0.9.9/builtin/providers/aws/resource_aws_subnet.go#L297

[s-urbaniak]

Possible flake countermeasure: increase the above 10 minute timeout.