Currently the libnetwork function `NewNetwork` does not allow
caller to pass a network ID and it is always generated internally.
This is sufficient for engine use. But it doesn't satisfy the needs
of libnetwork being used as an independent library in programs other
than the engine. This enhancement is one of the many needed to
facilitate a generic libnetwork.
Signed-off-by: Jana Radhakrishnan <mrjana@docker.com>