salt: Ensure we do not have multi cidr for Control Plane with MetalLB

Today we do not support multiple subnets for the Control Plane
network when MetalLB is enabled, so this commit check that it's
not the case

See: #3502

CHANGELOG.md

@@ -11,6 +11,12 @@
 - Fix UI issues in multi nodes environment when a node
   is unavailable (PR[#3521](https://github.com/scality/metalk8s/pull/3521))
 
+## Bug fixes
+
+- Enforce a single subnet for control plane when using a
+  MetalLB-managed VIP for Ingress
+  (PR [#3533](https://github.com/scality/metalk8s/pull/3533))
+
 ## Release 2.10.2
 ### Bug fixes
 - Fix the link to documentation from the UI navigation bar

salt/_pillar/metalk8s.py

@@ -94,13 +94,18 @@ def _load_networks(config_data):
     # MetalLB disabled by default
     networks_data["controlPlane"].setdefault("metalLB", {}).setdefault("enabled", False)
 
-    if networks_data["controlPlane"]["metalLB"]["enabled"] and not networks_data[
-        "controlPlane"
-    ].get("ingress", {}).get("ip"):
-        errors.append(
-            "'ip' for 'ingress' in 'controlPlane' network is mandatory when 'metalLB'"
-            "is enabled"
-        )
+    if networks_data["controlPlane"]["metalLB"]["enabled"]:
+        if not networks_data["controlPlane"].get("ingress", {}).get("ip"):
+            errors.append(
+                "'ip' for 'ingress' in 'controlPlane' network is mandatory when "
+                "'metalLB' is enabled"
+            )
+        if len(networks_data["controlPlane"]["cidr"]) > 1:
+            errors.append(
+                "Enabling 'metalLB' requires a single 'cidr' in "
+                "'controlPlane' network, see "
+                "https://github.com/scality/metalk8s/issues/3502"
+            )
 
     if errors:
         return __utils__["pillar_utils.errors_to_dict"](errors)