Implement HTTP-only mode

This is very useful for reserve proxies
in cases when TLS passthrough is not desired.

cli/flags.go

@@ -150,6 +150,12 @@ var (
 			EnvVars: []string{"PROFILING_ADDRESS"},
 			Value:   "localhost:9999",
 		},
+		&cli.BoolFlag{
+			Name:    "http-only-mode",
+			Usage:   "serve content directly via HTTP using the Host header to identify the repository",
+			EnvVars: []string{"HTTP_ONLY_MODE"},
+			Value:   false,
+		},
 
 		// ############################
 		// ### ACME Client Settings ###