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/test_framework/test_framework.py

@@ -111,6 +111,7 @@ class BitcoinTestFramework(metaclass=BitcoinTestMetaClass):
         # are not imported.
         self.wallet_names = None
         self.set_test_params()
+        assert self.wallet_names is None or len(self.wallet_names) <= self.num_nodes
         if self.options.timeout_factor == 0 :
             self.options.timeout_factor = 99999
         self.rpc_timeout = int(self.rpc_timeout * self.options.timeout_factor) # optionally, increase timeout by a factor
@@ -390,9 +391,13 @@ class BitcoinTestFramework(metaclass=BitcoinTestMetaClass):
                 assert_equal(chain_info["initialblockdownload"], False)
 
     def import_deterministic_coinbase_privkeys(self):
-        wallet_names = [self.default_wallet_name] * len(self.nodes) if self.wallet_names is None else self.wallet_names
-        assert len(wallet_names) <= len(self.nodes)
-        for wallet_name, n in zip(wallet_names, self.nodes):
+        for i in range(self.num_nodes):
+            self.init_wallet(i)
+
+    def init_wallet(self, i):
+        wallet_name = self.default_wallet_name if self.wallet_names is None else self.wallet_names[i] if i < len(self.wallet_names) else False
+        if wallet_name is not False:
+            n = self.nodes[i]
             if wallet_name is not None:
                 n.createwallet(wallet_name=wallet_name, descriptors=self.options.descriptors, load_on_startup=True)
             n.importprivkey(privkey=n.get_deterministic_priv_key().key, label='coinbase')