wallet: Make -wallet setting not create wallets

This changes -wallet setting to only load existing wallets, not create new ones.

- Fixes settings.json corner cases reported by sjors & promag:
  https://github.com/bitcoin-core/gui/issues/95,
  https://github.com/bitcoin/bitcoin/pull/19754#issuecomment-685858578,
  https://github.com/bitcoin/bitcoin/pull/19754#issuecomment-685858578

- Prevents accidental creation of wallets reported most recently by jb55
  http://www.erisian.com.au/bitcoin-core-dev/log-2020-09-14.html#l-355

- Simplifies behavior after #15454. #15454 took the big step of disabling
  creation of the default wallet. This PR extends it to avoid creating other
  wallets as well. With this change, new wallets just aren't created on
  startup, instead of sometimes being created, sometimes not. #15454 release
  notes are updated here and are simpler.

This change should be targeted for 0.21.0. It's a bug fix and simplifies
behavior of the #15937 / #19754 / #15454 features added in 0.21.0.

test/functional/tool_wallet.py

@@ -218,7 +218,8 @@ class ToolWalletTest(BitcoinTestFramework):
     def test_salvage(self):
         # TODO: Check salvage actually salvages and doesn't break things. https://github.com/bitcoin/bitcoin/issues/7463
         self.log.info('Check salvage')
-        self.start_node(0, ['-wallet=salvage'])
+        self.start_node(0)
+        self.nodes[0].createwallet("salvage")
         self.stop_node(0)
 
         self.assert_tool_output('', '-wallet=salvage', 'salvage')