Use GRPC url standard

[serathius]

Etcd code and tests depend on In particular our tests depend a lot on unix://localhost:port url schema, while
according to grpc standard: https://grpc.github.io/grpc/cpp/md_doc_naming.html the supported names are: unix:path or unix://absolute_path (so unix:///tmp/foo). This issue blocks grpc upgrade as etcd tests will not pass until we stop using unix://localhost:port.

Context #14104

First issue that we need to address is stop depending on unix://localhost:port schema in tests to unblock dependency upgrade. Later we should propose a plan to match grpc standard. I expect this might require multiple releases to avoid introducing breaking changes.

[serathius]

@ptabor Thanks for fixing the connection format, any followups needed or we can close the issue?

[serathius]

ping

[ptabor]

It's good enough for now. I don't think we have cycles to review all URL schemes proactively.