Allow the get-settings-upgrade-txs command to reuse deployed contracts

[sisuresh]

At the moment, get-settings-upgrade-txs gives you three transactions to submit - upload wasm tx, create contract tx, and invocation tx. If the contract has already been deployed in the past, then you only really need the invocation tx. There might be some merit to using a different contract per upgrade, but I don't think that's significant.