feat: pre-creation of nets

[samuelemusiani]

With large clusters the creation of a VNet and its propagation takes a lot of time. As the propagation phase will take almost the same amount of time regardless the number of VNets, we could pre-allocate a predefined number of VNets (like 10), and when a user requires a new Net we assign one of them to it without. After reaching a threshold (like 2), we add new VNets again in a single pass.