Add --namespace flag to Podman

Allows joining libpod to a specific namespace when running a
Podman command.

Signed-off-by: Matthew Heon <matthew.heon@gmail.com>

cmd/podman/main.go

@@ -172,6 +172,11 @@ func main() {
 			Usage: "log messages above specified level: debug, info, warn, error (default), fatal or panic",
 			Value: "error",
 		},
+		cli.StringFlag{
+			Name:  "namespace",
+			Usage: "set the libpod namespace, used create separate views of the containers and pods on the system",
+			Value: "",
+		},
 		cli.StringFlag{
 			Name:  "root",
 			Usage: "path to the root directory in which data, including images, is stored",