Fix broadcast_entropy function in verifier.py

[scaffino]

1. in the input parameters of the function, entropy_tx is a Transaction object and not a string. Use Transaction
2. if dry run, return test_txid at the beginning - then the more complex logic. This optimizes taking away the general stuff at the beginning and reading the more complex logic only if needed
3. broadcast the entropy tx to every node (for loop), and check that incoming data (i.e., the transaction id) is valid. One way is to check it against the locally computed txid
4. assert txid is not None, "Entropy not posted" --> this line now fails. Use error handling

[scaffino]

Done, but needs testing as soon as the testnet node is synched up