config_network_type: set macvlan default mode to private

If a default mode is not set, the container requires an explicit
mode specified in the config file, otherwise creating the
container fails.

Signed-off-by: Bogdan Purcareata <bogdan.purcareata@freescale.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>