Add option to configure template job HCL file

Previously, the template job HCL file was hardcoded using go:embed
in the binary. However, this did not allow users running Poseidon
to change its content. Now, users can change the content of the
template job HCL file using the configuration option.

internal/config/config.go

@@ -28,6 +28,7 @@ var (
 				KeyFile:  "",
 			},
 			InteractiveStderr: true,
+			TemplateJobFile:   "",
 		},
 		Nomad: Nomad{
 			Address: "127.0.0.1",
@@ -63,6 +64,7 @@ type server struct {
 	Token             string
 	TLS               TLS
 	InteractiveStderr bool
+	TemplateJobFile   string
 }
 
 // URL returns the URL of the Poseidon webserver.