Port/fix txnmall.sh regression test

Ported txnmall.sh to Python, and updated to match
recent transaction malleability changes.

I also modified it so it tests both double-spending
confirmed and unconfirmed (only-in-mempool) transactions.

Renamed to txn_doublespend, since that is really what is
being tested. And told the pull-tester to run both
variations on this test.
Truthier error message when rpcpassword is missing
Port of wallet.sh to python (wallet.py).

Also included are minor edits to util.py to create a clean blockchain and add a parameter to gather_inputs to specify number of confirmations.
Edited rpc-tests to run python script not shell script.
Merge pull request #5317

8656dbb Port/fix txnmall.sh regression test (Gavin Andresen)
Merge pull request #5333

e4ef724 Edited rpc-tests to run python script not shell script. (mrbandrews)
189fb52 Port of wallet.sh to python (wallet.py). (mrbandrews)
Merge pull request #5318

77c38bb Truthier error message when rpcpassword is missing (Glenn Willen)