feat: stagger startup of containers on Azure

Seeing some cases where starting up large (80+ workers) load tests
on ACI seems to exceed some underlying Azure Blob Storage limits
with multiple containers all accessing the same blob container
concurrently.

With this change Artillery will pause for up to 10 seconds after
launching every batch of 10 workers.

packages/artillery/lib/platform/az/aci.js

@@ -320,6 +320,15 @@ class PlatformAzureACI {
       const { workerId } = await this.createWorker();
       this.workers[workerId] = { workerId };
       await this.runWorker(workerId);
+
+      if (i > 0 && i % 10 === 0) {
+        const delayMs =
+          Math.floor(
+            Math.random() *
+              parseInt(process.env.AZURE_LAUNCH_STAGGER_SEC || '10', 10)
+          ) * 1000;
+        await sleep(delayMs);
+      }
     }
 
     let instancesCreated = false;