Add pool of preconfigured networks for sandboxes

[ValentaTomas]

When creating a lot of sandboxes at the same time the biggest bottleneck right now is the network setup that takes more and more time depending on the number of networks being created. When creating 20+ sandboxes the additional delay starts to cross 1+ seconds.

The solution here is to create a pool of pre-created networks that we can then immediately use while refilling the pool.

[ValentaTomas]

We added the pool of preconfigured networks. Adding a pool for the FC was more complicated.