add explicit networking mode for cases where subnet autodetection logic fails

In some cases, when an account have multiples subnets tagged with "z1" and "dev" the module fails to correctly autodetect the subnet.

this merge request add an "explicit networking" mode to override subnet/az autodetection.

This is indended to be the last 4.X.X version of the module, starting with version 5.0.0, the networking parameters will be explicit only.