Support for private nodes on existing networks

[fboucquez]

Atm, bootstrap has been well tested for both brand new private networks and nodes on the existing testnet network.

This task to improve the presets to facilitate how admins can create new nodes on existing private networks. It could just be a new empty preset or an empty assembly on the bootstrap preset. The user would need to extend with a custom preset and provided seed.

https://github.com/nemtech/symbol-docs/issues/542