cmd/swarm: Exit if --ethapi is set

The previous attempt to use --ethapi as a fallback if --ens-api is not
set does not work because --ens-api has a default value, and also
setting --ens-api to "" is the suggested way to disable ENS lookups.

Signed-off-by: Lewis Marshall <lewis@lmars.net>

swarm/dev/scripts/boot-cluster.sh

@@ -208,7 +208,7 @@ start_swarm_node() {
     --bootnodes    "${BOOTNODE_URL}"
     --datadir      "${dir}"
     --identity     "${name}"
-    --ethapi       "${GETH_RPC_URL}"
+    --ens-api      "${GETH_RPC_URL}"
     --bzznetworkid "321"
     --bzzaccount   "${address}"
     --password     "${dir}/password"