Add Support for Manta Object Storage as a Gateway (#5025)

Manta is an Object Storage by [Joyent](https://www.joyent.com/)

This PR adds initial support for Manta. It is intended as non-production 
ready so that feedback can be obtained.

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

@@ -0,0 +1,29 @@
+package authentication
+
+import (
+	"crypto/md5"
+	"fmt"
+	"strings"
+
+	"golang.org/x/crypto/ssh"
+)
+
+// formatPublicKeyFingerprint produces the MD5 fingerprint of the given SSH
+// public key. If display is true, the fingerprint is formatted with colons
+// between each byte, as per the output of OpenSSL.
+func formatPublicKeyFingerprint(key ssh.PublicKey, display bool) string {
+	publicKeyFingerprint := md5.New()
+	publicKeyFingerprint.Write(key.Marshal())
+	publicKeyFingerprintString := fmt.Sprintf("%x", publicKeyFingerprint.Sum(nil))
+
+	if !display {
+		return publicKeyFingerprintString
+	}
+
+	formatted := ""
+	for i := 0; i < len(publicKeyFingerprintString); i = i + 2 {
+		formatted = fmt.Sprintf("%s%s:", formatted, publicKeyFingerprintString[i:i+2])
+	}
+
+	return strings.TrimSuffix(formatted, ":")
+}