We'd like to start using the DNS load balancer in the latest version of gRPC. That means putting all IPs for a service under a single hostname (or using a SRV record, but we're not taking that path). This change adds an sd-test-srv to act as our service discovery DNS service. It returns both Boulder IP addresses for any A lookup ending in ".boulder". This change also sets up the Docker DNS for our boulder container to defer to sd-test-srv when it doesn't know an answer.
sd-test-srv doesn't know how to resolve public Internet names like `github.com`. Resolving public names is required for the `godep-restore` test phase, so this change breaks out a copy of the boulder container that is used only for `godep-restore`.
This change implements a shim of a DNS resolver for gRPC, so that we can switch to DNS-based load balancing with the currently vendored gRPC, then when we upgrade to the latest gRPC we won't need a simultaneous config update.
Also, this change introduces a check at the end of the integration test that each backend received at least one RPC, ensuring that we are not sending all load to a single backend.
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