No longer create a default wallet. The default wallet will still be
loaded if it exists and not other wallets were specified (anywhere,
including settings.json, bitcoin.conf, and command line).
Tests are updated to be started with -wallet= if they need the default
Added test to wallet_startup.py testing that no default wallet is
created and that it is loaded if it exists and no other wallets were
Tell users how to load or create a wallet when no wallet is loaded
Depends on D10240