enable using ephemeral disks by default (#461)

src/api-service/__app__/onefuzzlib/azure/vmss.py

@@ -208,6 +208,7 @@ def create_vmss(
     image: str,
     network_id: str,
     spot_instances: bool,
+    ephemeral_os_disks: bool,
     extensions: List[Any],
     password: str,
     ssh_public_key: str,
@@ -259,7 +260,9 @@ def create_vmss(
         },
         "virtual_machine_profile": {
             "priority": "Regular",
-            "storage_profile": {"image_reference": image_ref},
+            "storage_profile": {
+                "image_reference": image_ref,
+            },
             "os_profile": {
                 "computer_name_prefix": "node",
                 "admin_username": "onefuzz",
@@ -300,6 +303,13 @@ def create_vmss(
             },
         }
 
+    if ephemeral_os_disks:
+        params["virtual_machine_profile"]["storage_profile"]["os_disk"] = {
+            "diffDiskSettings": {"option": "Local"},
+            "caching": "ReadOnly",
+            "createOption": "FromImage",
+        }
+
     if spot_instances:
         # Setting max price to -1 means it won't be evicted because of
         # price.