proxy_fastly: get max number of backends from config

app/terraform/proxy/fastly.py

@@ -1,5 +1,7 @@
 from typing import Any, Optional
 
+from flask import current_app
+
 from app.extensions import db
 from app.models.mirrors import Proxy
 from app.terraform.proxy import ProxyAutomation
@@ -9,7 +11,7 @@ class ProxyFastlyAutomation(ProxyAutomation):
     short_name = "proxy_fastly"
     description = "Deploy proxies to Fastly"
     provider = "fastly"
-    subgroup_max = 5
+    subgroup_max = max(current_app.config.get("FASTLY_MAX_BACKENDS", 5), 20)
 
     template_parameters = [
         "aws_access_key",