main: Temporarily disable "terraform login" as a command

We're not ready to ship this in a release yet because there's still some
remaining work to do on the Terraform Cloud side, but we want to get the
implementation work behind this into the master branch so it's easier to
maintain it in the mean time, rather than letting this long-lived branch
live even longer.

We'll continue to iterate on UX polish and other details in subsequent
commits, and eventually enable this.

commands.go

@@ -180,11 +180,15 @@ func initCommands(config *Config, services *disco.Disco) {
 			}, nil
 		},
 
+		// "terraform login" is disabled until Terraform Cloud is ready to
+		// support it.
+		/*
 			"login": func() (cli.Command, error) {
 				return &command.LoginCommand{
 					Meta: meta,
 				}, nil
 			},
+		*/
 
 		"output": func() (cli.Command, error) {
 			return &command.OutputCommand{