fix: allow workload script to be disabled (#59)

The apm-integration-testing project will set the `WORKLOAD_DISABLED`
environment variable based on the `--no-opbeans-node-loadgen` command
line flag.

ecosystem-workload.config.js

@@ -1,13 +1,21 @@
-module.exports = {
+'use strict'
+
+const config = {
   apps: [{
+    name: 'server',
+    script: './server.js',
+    instances: 1
+  }]
+}
+
+if (process.env.WORKLOAD_DISABLED !== 'True') {
+  config.apps.push({
     name: 'workload',
     script: './node_modules/.bin/workload',
     args: '-f .workload.js',
     instances: 1,
     restart_delay: 2000
-  }, {
-    name: 'server',
-    script: './server.js',
-    instances: 1
-  }]
+  })
 }
+
+module.exports = config