*: support getting role in auth v3

This commit implements RoleGet() RPC of etcdserver and adds a new
subcommand "role get" to etcdctl v3. It will list up permissions that
are granted to a given role.

$ ETCDCTL_API=3 bin/etcdctl role get r1
Role r1
KV Read:
        b
        d
KV Write:
        a
        c
        d

etcdserver/api/v3rpc/auth.go

@@ -66,8 +66,11 @@ func (as *AuthServer) RoleDelete(ctx context.Context, r *pb.AuthRoleDeleteReques
 }
 
 func (as *AuthServer) RoleGet(ctx context.Context, r *pb.AuthRoleGetRequest) (*pb.AuthRoleGetResponse, error) {
-	plog.Info("not implemented yet")
-	return nil, nil
+	resp, err := as.authenticator.RoleGet(ctx, r)
+	if err != nil {
+		return nil, togRPCError(err)
+	}
+	return resp, nil
 }
 
 func (as *AuthServer) RoleRevoke(ctx context.Context, r *pb.AuthRoleRevokeRequest) (*pb.AuthRoleRevokeResponse, error) {