Add --default-mounts-file hidden flag

The hidden flag is used to override the path of the default mounts file
for testing purposes.
Also modified the secrets pkg to allow for this override to happen.

Signed-off-by: umohnani8 <umohnani@redhat.com>

Closes: #678
Approved by: mheon

cmd/podman/main.go

@@ -124,6 +124,11 @@ func main() {
 			Name:  "cpu-profile",
 			Usage: "path for the cpu profiling results",
 		},
+		cli.StringFlag{
+			Name:   "default-mounts-file",
+			Usage:  "path to default mounts file",
+			Hidden: true,
+		},
 		cli.StringFlag{
 			Name:   "hooks-dir-path",
 			Usage:  "set the OCI hooks directory path",