Drastically reduce Koschei L2 cache capacity

This is required to free some disk space until we migrate to bigger
storage.

inventory/group_vars/koschei-backend

@@ -1,5 +1,6 @@
 ---
 # Define resources for this group of hosts here.
+# FIXME increase lvm_size at the earliest opportunity
 lvm_size: 20000
 mem_size: 4096
 num_cpus: 4

inventory/group_vars/koschei-backend-stg

@@ -1,5 +1,6 @@
 ---
 # Define resources for this group of hosts here.
+# FIXME increase lvm_size at the earliest opportunity
 lvm_size: 20000
 mem_size: 4096
 num_cpus: 4

roles/koschei/backend/templates/config-backend.cfg.j2

@@ -36,7 +36,8 @@ config = {
         "resolver_queue_size": 100,
         "dependency_fetch_chunk_size": 150,
         "dependency_cache_capacity": 50000,
-        "cache_l2_capacity": 42,
+        # FIXME increase cache_l2_capacity once there is more disk space available
+        "cache_l2_capacity": 16,
         "ignore_weak_deps": True,
     },
     "services": {