clean up demo bash scripts

* allow other level of RUST logging
 * avoid "echo" in favor of printf (builtin)
 * single quotes for literals, double quotes for variables

multinode-demo/leader.sh

@@ -1,4 +1,7 @@
 #!/bin/bash
-export RUST_LOG=solana=info
+
+# if RUST_LOG is unset, default to info
+export RUST_LOG=${RUST_LOG:-solana=info}
+
 sudo sysctl -w net.core.rmem_max=26214400
 cargo run --release --bin solana-fullnode -- -l leader.json < genesis.log