podman: allow to override Tmpdir

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

Closes: #871
Approved by: mheon

cmd/podman/main.go

@@ -153,6 +153,10 @@ func main() {
 			Name:  "root",
 			Usage: "path to the root directory in which data, including images, is stored",
 		},
+		cli.StringFlag{
+			Name:  "tmpdir",
+			Usage: "path to the tmp directory",
+		},
 		cli.StringFlag{
 			Name:  "runroot",
 			Usage: "path to the 'run directory' where all state information is stored",