Only configure networking when creating a net ns

When joining an existing namespace, don't default to configuring a
loopback interface in that namespace.

Its creator should have done that, and we don't want to fail to create
the container when we don't have sufficient privileges to configure the
network namespace.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

libcontainer/specconv/spec_linux.go

@@ -219,7 +219,7 @@ func CreateLibcontainerConfig(opts *CreateOpts) (*configs.Config, error) {
 			}
 			config.Namespaces.Add(t, ns.Path)
 		}
-		if config.Namespaces.Contains(configs.NEWNET) {
+		if config.Namespaces.Contains(configs.NEWNET) && config.Namespaces.PathOf(configs.NEWNET) == "" {
 			config.Networks = []*configs.Network{
 				{
 					Type: "loopback",