Current code that pass gateways to vz sdk is not suitable for
updates. If update has no gateways while we had them before
we need to pass "" for vz sdk gateways to reset old value.

The code definitely deserves its own function.

Drop checks that skip setting gateways if network address
is not set. Such a configuration is possible in vz sdk.
Signed-off-by: NNikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>