gateway/manta: Add support for RBAC (#5332)

Manta has the ability to allow users to authenticate with a 
username other than the main account. We want to expose 
this functionality to minio manta gateway.

vendor/github.com/joyent/triton-go/triton.go

@@ -14,5 +14,6 @@ type ClientConfig struct {
 	TritonURL   string
 	MantaURL    string
 	AccountName string
+	Username    string
 	Signers     []authentication.Signer
 }