Skip VPC endpoint routes when removing default route table's routes

[ewbankkit]

Fixes #10295.
Added an acceptance test:

$ make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSDefaultRouteTable_'

[stack72]

This LGTM! Thanks for the work here @ewbankkit

% make testacc TEST=./builtin/providers/aws TESTARGS='-run=estAccAWSDefaultRouteTable_'           1 ↵
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2016/11/23 13:05:08 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=estAccAWSDefaultRouteTable_ -timeout 120m
=== RUN   TestAccAWSDefaultRouteTable_basic
--- PASS: TestAccAWSDefaultRouteTable_basic (91.49s)
=== RUN   TestAccAWSDefaultRouteTable_swap
--- PASS: TestAccAWSDefaultRouteTable_swap (116.96s)
=== RUN   TestAccAWSDefaultRouteTable_vpc_endpoint
--- PASS: TestAccAWSDefaultRouteTable_vpc_endpoint (72.97s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	281.428s

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