ssl: Set a global boolean to enable SSL across Minio (#3558)

We have been using `isSSL()` everywhere we can set
a global value once and re-use it again.

cmd/server-startup-msg.go

@@ -67,7 +67,7 @@ func printStartupMessage(apiEndPoints []string) {
 
 	// SSL is configured reads certification chain, prints
 	// authority and expiry.
-	if isSSL() {
+	if globalIsSSL {
 		certs, err := readCertificateChain()
 		fatalIf(err, "Unable to read certificate chain.")
 		printCertificateMsg(certs)