Add NETWORK_ID variable (#2330)

Same variable BOOTNODE_PORT was used for p2p port of bootnode and testnet Chain and Network ID. Adding variable NETWORK_ID to make scripts less confusing and create option to choose arbitrary ID. 

Co-authored-by: Mário Havel <61149543+taxmeifyoucan@users.noreply.github.com>
Co-authored-by: Michael Sproul <michael@sigmaprime.io>

scripts/local_testnet/ganache_test_node.sh

@@ -9,5 +9,5 @@ ganache-cli \
 	--mnemonic "$ETH1_NETWORK_MNEMONIC" \
 	--port 8545 \
 	--blockTime 3 \
-	--networkId 4242 \
-	--chainId 4242
+	--networkId "$NETWORK_ID" \
+	--chainId "$NETWORK_ID"