provider/aws: Export internal tunnel addresses + document

[rdark]

Use case: wanted to plug in a packer-based router configuration build pipeline.

These addresses are required in order to configure the tunnel

[stack72]

LGTM! :)

% make testacc TEST=./builtin/providers/aws/ TESTARGS='-run=TestAccAWSVpnConnection_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/31 20:38:57 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws/ -v -run=TestAccAWSVpnConnection_ -timeout 120m
=== RUN   TestAccAWSVpnConnection_importBasic
--- PASS: TestAccAWSVpnConnection_importBasic (245.02s)
=== RUN   TestAccAWSVpnConnection_basic
--- PASS: TestAccAWSVpnConnection_basic (641.73s)
=== RUN   TestAccAWSVpnConnection_withoutStaticRoutes
--- PASS: TestAccAWSVpnConnection_withoutStaticRoutes (214.87s)
=== RUN   TestAccAWSVpnConnection_disappears
--- PASS: TestAccAWSVpnConnection_disappears (205.14s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	1306.785s

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