agent: add optional param to -dev flag for connect (#6126)

Consul Connect must route traffic between network namespaces through a
public interface (i.e. not localhost). In order to support testing in
dev mode, users needed to manually set the interface which doesn't
make for a smooth experience.

This commit adds a facility for adding optional parameters to the
`nomad agent -dev` flag and uses it to add a `-dev=connect` flag that
binds to a public interface on the host.

command/agent/command.go

@@ -56,7 +56,7 @@ type Command struct {
 }
 
 func (c *Command) readConfig() *Config {
-	var dev bool
+	var dev *devModeConfig
 	var configPath []string
 	var servers string
 	var meta []string
@@ -77,7 +77,10 @@ func (c *Command) readConfig() *Config {
 	flags.Usage = func() { c.Ui.Error(c.Help()) }
 
 	// Role options
-	flags.BoolVar(&dev, "dev", false, "")
+	flags.Var((flaghelper.FuncOptionalStringVar)(func(s string) (err error) {
+		dev, err = newDevModeConfig(s)
+		return err
+	}), "dev", "")
 	flags.BoolVar(&cmdConfig.Server.Enabled, "server", false, "")
 	flags.BoolVar(&cmdConfig.Client.Enabled, "client", false, "")
 
@@ -204,8 +207,8 @@ func (c *Command) readConfig() *Config {
 
 	// Load the configuration
 	var config *Config
-	if dev {
-		config = DevConfig()
+	if dev != nil {
+		config = DevConfig(dev)
 	} else {
 		config = DefaultConfig()
 	}
@@ -1164,7 +1167,13 @@ General Options (clients and servers):
     Start the agent in development mode. This enables a pre-configured
     dual-role agent (client + server) which is useful for developing
     or testing Nomad. No other configuration is required to start the
-    agent in this mode.
+    agent in this mode, but you may pass an optional comma-separated
+    list of mode configurations:
+
+    -dev=connect
+      Start the agent in development mode, but bind to a public network
+      interface rather than localhost for using Consul Connect. This
+      mode is supported only on Linux as root.
 
 Server Options: