[feature request] ipam driver null support

[tpreclik]

I would like to use the plugin in passthrough mode with a null ipam driver. However, the joining of a container to such a network fails:

Error response from daemon: NetworkDriver.Join: Parse gateway [] error: invalid CIDR address:

The problem is that Join tries to parse the gateway address and statically sets DisableGatewayService to false:

docker-sriov-plugin/driver/driver.go

Line 413 in b8d6f3f

gw, _, err := net.ParseCIDR(genNw.IPv4Data.Gateway)