When there is no account/password specified for swarm, there was an error shown in the logs (asking for address/password), and the swarm process would quit.
This PR changes the behaviour so that if a swarm address/password are not specified in the config, it attempts to use the blockchain address/password specified in `config/blockchain > account`. If `config/blockchain > account > address` doesn’t exist, the first account controlled by the node is used (provided by `web3.eth.getAccounts`, along with the password from `config/blockchain > account > password`.