Add multisig modification to nemesis accounts.

[fboucquez]

Add nemesis block multisig modification to symbol-network, so nodes' Mainnet accounts, faucet, and founder accounts are multisig from block 0. I had that for Mainnet/Testnet configuration, but I need to make it nice for the cli tool. This feature depends on symbol-signer-cli to create, cosign and merge them into nemesis block transactions. Of course, multisig can be done after network launches too!