[WIP] aws_vpn_connection: fix possible remote state data inconsistency

[n3ph]

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Closes #11293

Release note for CHANGELOG:

aws_vpn_connection: fix possible remote state data inconsistency 

Output from acceptance testing:

After substantiated test case (c65aa11):

make testacc TEST=./aws TESTARGS='-run=TestAWSVpnConnection_xmlconfig'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws -v -count 1 -parallel 20 -run=TestAWSVpnConnection_xmlconfig -timeout 120m
=== RUN   TestAWSVpnConnection_xmlconfig
--- FAIL: TestAWSVpnConnection_xmlconfig (0.00s)
    resource_aws_vpn_connection_test.go:397: First address from tunnel XML was incorrect.
FAIL
FAIL	github.com/terraform-providers/terraform-provider-aws/aws	0.019s
FAIL
make: *** [GNUmakefile:24: testacc] Error 1

[ghost]

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