avoid overwriting validator balances

[bernardoaraujor]

We need to use polkadot-local chain for polkadot-stps.

The project also relies on the feature of setting genesis balances.

We're running into a problem where the validator balances are also being overwritten:

   "Thread 'main' panicked at 'Stash does not have enough balance to bond.', /Users/pepo/.cargo/git/checkouts/substrate-7e08433d4c370a21/20037d5/frame/staking/src/pallet/mod.rs:578\n" +

No blocks are even produced on the chain.

Steps to reproduce:

1. Clone polkadot-stps
2. Apply this change:

diff --git a/tests/relay-single-node-native.json b/tests/relay-single-node-native.json
index 538dc2d..9afb081 100644
--- a/tests/relay-single-node-native.json
+++ b/tests/relay-single-node-native.json
@@ -5,14 +5,12 @@
     },
     "relaychain": {
       "default_image": "docker.io/parity/polkadot:latest",
-      "chain": "rococo-local",
+      "chain": "polkadot-local",
       "default_command": "./polkadot",
       "genesis": {
         "runtime": {
-          "runtime_genesis_config": {
-            "balances": {
-              "balances": {% include "./funded-accounts.json" %}
-            }
+          "balances": {
+            "balances": {% include "./funded-accounts.json" %}
           }
         }
       },

3. Run:

./polkadot-stps.sh test_native tests/relay-single-node-native.zndsl

---

It's weird that this feature works with rococo-local but not polkadot-local, so this could perhaps be a bug?

One possible solution discussed with @pepoviola was to add the possibility of zombienet supporting the use of append_balances: <string to file> instead of balances. It would avoid overwriting any validator balances.