embed: explicitly define unix proto

[hexfusion]

This is pending grpc/grpc-go#1883

ref: #9340

[hexfusion]

Upstream grpc-go fixes will be merged next week allowing unix support to work.

[hexfusion]

@gyuho PTAL.

[hexfusion]

WARNING: 2018/04/10 10:53:42 grpc: addrConn.resetTransport failed to create client transport: connection error: desc = "transport: Error while dialing dial tcp: address unix://localhost:15452000000: too many colons in address"; Reconnecting to {unix://localhost:15452000000 0 }

I need to understand how this can happen, seems like tests are not using patched grpc-go.

[gyuho]

@hexfusion etcd master still uses old gRPC, so we may see it for awhile :0 Also we need to wait until we upgrade gRPC in master branch including your fix on unix socket.

[gyuho]

Let's make it simpler?

if sctx.network == "unix" {
    addr = fmt.Sprintf("%s://%s", network, addr)
}

[hexfusion]

sure, all set

[gyuho]

lgtm thanks @hexfusion !

[hexfusion]

@gyuho anything else we need here? I plan on improving test coverage in another PR. Thanks!