Split network from sub-systems

[eboadom]

Currently, with yarn generate you are obliged to choose a pool as target, but some proposals don't really deal with pools, only with an specific network.

One idea would be having 2 separate stages on the cli prompt:

1. Networks. The user chooses 1-to-n networks target.
2. Sub-systems. The user chooses if the proposal is targetting an Aave pool, or any other sub-system, leaving an "empty" option for when the proposal is too heterogeneous. In this last scenario, using CommonTestBase on the tests, and everything pretty much generic.