Add fallbacks when NAMESPACE, RELAY_CHAIN_RPC_URL or RPC_NODE_URL_PATTERN are not set

[PierreBesson]

Some base env vars are currently required to start the testnet-manager properly but I think we should be able to remove this requirements which would make the app work out of the box when deployed to a namespace which contains running nodes deployed with our helm-chart.

• NAMESPACE: call this variable RELAY_CHAIN_NAME instead which would allow having a chain name different from the namespace name then use the content of /var/run/secrets/kubernetes.io/serviceaccount/namespace as default
• RELAY_CHAIN_RPC_URL: find pods running with the label chain=RELAY_CHAIN_NAME then try to query it
• RPC_NODE_URL_PATTERN: use "NODE_NAME:9944", it is possible now as we no longer have to support both 9933/9944