grpc: send client traffic through envoy (#2469)

* wip

* wip

* handle wildcards in override name

* remove wait for ready, add comment about sync, force initial sync complete in test

* address comments

internal/netutil/netutil.go

@@ -0,0 +1,22 @@
+// Package netutil contains various functions that help with networking.
+package netutil
+
+import "net"
+
+// AllocatePorts allocates random ports suitable for listening.
+func AllocatePorts(count int) ([]string, error) {
+	var ports []string
+	for i := 0; i < count; i++ {
+		li, err := net.Listen("tcp", "127.0.0.1:0")
+		if err != nil {
+			return nil, err
+		}
+		_, port, _ := net.SplitHostPort(li.Addr().String())
+		err = li.Close()
+		if err != nil {
+			return nil, err
+		}
+		ports = append(ports, port)
+	}
+	return ports, nil
+}