getrawchangeaddress should fail when keypool exhausted

An user on IRC reported an issue where `getrawchangeaddress`
keeps returning a single address when the keypool is exhausted.
In my opinion this is strange behaviour.

- Change CReserveKey to fail when running out of keys in the keypool.
- Make `getrawchangeaddress` return RPC_WALLET_KEYPOOL_RAN_OUT when
  unable to create an address.
- Add a Python RPC test for checking the keypool behaviour in combination
  with encrypted wallets.

src/wallet.cpp

@@ -2010,11 +2010,7 @@ bool CReserveKey::GetReservedKey(CPubKey& pubkey)
         if (nIndex != -1)
             vchPubKey = keypool.vchPubKey;
         else {
-            if (pwallet->vchDefaultKey.IsValid()) {
-                LogPrintf("CReserveKey::GetReservedKey(): Warning: Using default key instead of a new key, top up your keypool!");
-                vchPubKey = pwallet->vchDefaultKey;
-            } else
-                return false;
+            return false;
         }
     }
     assert(vchPubKey.IsValid());