Fix service_attachment example to use server-generated ip address

[modular-magician]

fixes hashicorp/terraform-provider-google#14080

I'm not exactly sure why we were using a hardcoded IP address in this example/test, but it's possible we need it (the project numbers are also hardcoded and we do appear to need those). It appears the cause of the error is an address from a previous test was not deleted, so we probably need to make sure the sweepers are working as intended. In this change, I'm attempting to make the IP address server-generated so that the test is slightly less brittle.

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Derived from GoogleCloudPlatform/magic-modules#7529