Simplify running cmd.Main() for external tests (#3636)

An external test that runs cmd.Main() has a difficulty to set cmd arguments
and MINIO_{ACCESS,SECRET}_KEY values, this commit changes a little the current
behavior in a way that helps external tests.

cmd/globals.go

@@ -110,11 +110,11 @@ var (
 	// Minio server user agent string.
 	globalServerUserAgent = "Minio/" + ReleaseTag + " (" + runtime.GOOS + "; " + runtime.GOARCH + ")"
 
-	// Access key passed from the environment
-	globalEnvAccessKey = os.Getenv("MINIO_ACCESS_KEY")
+	// Global server's access key
+	globalEnvAccessKey = ""
 
-	// Secret key passed from the environment
-	globalEnvSecretKey = os.Getenv("MINIO_SECRET_KEY")
+	// Global server's secret key
+	globalEnvSecretKey = ""
 
 	// url.URL endpoints of disks that belong to the object storage.
 	globalEndpoints = []*url.URL{}