[security] rpc: Do not transfer access/secret key. (#4857)

This is an improvement upon existing implementation
by avoiding transfer of access and secret keys over
the network. This change only exchanges JWT tokens
generated by an rpc client. Even if the JWT can be
traced over the network on a non-TLS connection, this
change makes sure that we never really expose the
secret key over the network.

cmd/rpc-common.go

@@ -60,8 +60,7 @@ type AuthRPCReply struct{}
 
 // LoginRPCArgs - login username and password for RPC.
 type LoginRPCArgs struct {
-	Username    string
-	Password    string
+	AuthToken   string
 	Version     string
 	RequestTime time.Time
 }
@@ -80,11 +79,8 @@ func (args LoginRPCArgs) IsValid() error {
 	return nil
 }
 
-// LoginRPCReply - login reply provides generated token to be used
-// with subsequent requests.
-type LoginRPCReply struct {
-	AuthToken string
-}
+// LoginRPCReply - login reply is a dummy struct perhaps for future use.
+type LoginRPCReply struct{}
 
 // LockArgs represents arguments for any authenticated lock RPC call.
 type LockArgs struct {