Classic peer discovery: node list warnings

Log warnings when:
- Local node is not present. Even though we force it on the node
list, this will not work for other cluster nodes if they have
the same list.
- There are duplicated nodes

(cherry picked from commit 647d65b)

Author: dcorbacho

deps/rabbit/src/rabbit_peer_discovery_classic_config.erl

@@ -21,8 +21,12 @@
 list_nodes() ->
     case application:get_env(rabbit, cluster_nodes, {[], disc}) of
         {Nodes, NodeType} ->
+            check_local_node(Nodes),
+            check_duplicates(Nodes),
             {ok, {add_this_node(Nodes), NodeType}};
         Nodes when is_list(Nodes) ->
+            check_local_node(Nodes),
+            check_duplicates(Nodes),
             {ok, {add_this_node(Nodes), disc}}
     end.
 
@@ -33,6 +37,26 @@ add_this_node(Nodes) ->
         false -> [ThisNode | Nodes]
     end.
 
+check_duplicates(Nodes) ->
+    case (length(lists:usort(Nodes)) == length(Nodes)) of
+        true ->
+            ok;
+        false ->
+            rabbit_log:warning("Classic peer discovery backend: list of "
+                               "nodes contains duplicates ~0tp",
+                               [Nodes])
+    end.
+
+check_local_node(Nodes) ->
+    case lists:member(node(), Nodes) of
+        true ->
+            ok;
+        false ->
+            rabbit_log:warning("Classic peer discovery backend: list of "
+                               "nodes does not contain the local node ~0tp",
+                               [Nodes])
+    end.
+
 -spec lock(Nodes :: [node()]) ->
     {ok, {{ResourceId :: string(), LockRequesterId :: node()}, Nodes :: [node()]}} |
     {error, Reason :: string()}.