Maintain order of configured SASL mechanisms

RabbitMQ should advertise the SASL mechanisms in the order as
configured in `rabbitmq.conf`.

Starting RabbitMQ with the following `rabbitmq.conf`:
```
auth_mechanisms.1 = PLAIN
auth_mechanisms.2 = AMQPLAIN
auth_mechanisms.3 = ANONYMOUS
```

translates prior to this commit to:
```
1> application:get_env(rabbit, auth_mechanisms).
{ok,['ANONYMOUS','AMQPLAIN','PLAIN']}
```

and after this commit to:
```
1> application:get_env(rabbit, auth_mechanisms).
{ok,['PLAIN','AMQPLAIN','ANONYMOUS']}
```

In our 4.0 docs we write:
> The server mechanisms are ordered in decreasing level of preference.

which complies with https://docs.oasis-open.org/amqp/core/v1.0/os/amqp-core-security-v1.0-os.html#type-sasl-mechanisms

deps/rabbit/priv/schema/rabbit.schema

@@ -455,10 +455,11 @@ end}.
     {datatype, atom}]}.
 
 {translation, "rabbit.auth_mechanisms",
-fun(Conf) ->
-    Settings = cuttlefish_variable:filter_by_prefix("auth_mechanisms", Conf),
-    [ V || {_, V} <- Settings ]
-end}.
+ fun(Conf) ->
+         Settings = cuttlefish_variable:filter_by_prefix("auth_mechanisms", Conf),
+         Sorted = lists:keysort(1, Settings),
+         [V || {_, V} <- Sorted]
+ end}.
 
 
 %% Select an authentication backend to use. RabbitMQ provides an

deps/rabbit/test/config_schema_SUITE_data/rabbit.snippets

@@ -239,6 +239,20 @@ default_permissions.write = .*",
   [{rabbit,
        [{anonymous_login_user, none}]}],
   []},
+
+ {auth_mechanisms_ordered,
+  "auth_mechanisms.1 = PLAIN
+auth_mechanisms.2 = AMQPLAIN
+auth_mechanisms.3 = ANONYMOUS",
+  [],
+  [{rabbit,
+    %% We expect the mechanisms in the order as declared.
+    [{auth_mechanisms, ['PLAIN', 'AMQPLAIN', 'ANONYMOUS']}]
+   }],
+  [],
+  nosort
+ },
+
  {cluster_formation,
   "cluster_formation.peer_discovery_backend = rabbit_peer_discovery_classic_config
 cluster_formation.classic_config.nodes.peer1 = rabbit@hostname1

deps/rabbitmq_ct_helpers/src/rabbit_ct_config_schema.erl

@@ -25,10 +25,14 @@ run_snippets(Config) ->
     {ok, [Snippets]} = file:consult(?config(conf_snippets, Config)),
     ct:pal("Loaded config schema snippets: ~tp", [Snippets]),
     lists:map(
-        fun({N, S, C, P})    -> ok = test_snippet(Config, {snippet_id(N), S, []}, C, P);
-           ({N, S, A, C, P}) -> ok = test_snippet(Config, {snippet_id(N), S, A},  C, P)
-        end,
-        Snippets),
+      fun({N, S, C, P}) ->
+              ok = test_snippet(Config, {snippet_id(N), S, []}, C, P, true);
+         ({N, S, A, C, P}) ->
+              ok = test_snippet(Config, {snippet_id(N), S, A},  C, P, true);
+         ({N, S, A, C, P, nosort}) ->
+              ok = test_snippet(Config, {snippet_id(N), S, A},  C, P, false)
+      end,
+      Snippets),
     ok.
 
 snippet_id(N) when is_integer(N) ->
@@ -40,7 +44,7 @@ snippet_id(A) when is_atom(A) ->
 snippet_id(L) when is_list(L) ->
     L.
 
-test_snippet(Config, Snippet = {SnipID, _, _}, Expected, _Plugins) ->
+test_snippet(Config, Snippet = {SnipID, _, _}, Expected, _Plugins, Sort) ->
     {ConfFile, AdvancedFile} = write_snippet(Config, Snippet),
     %% We ignore the rabbit -> log portion of the config on v3.9+, where the lager
     %% dependency has been dropped
@@ -50,8 +54,12 @@ test_snippet(Config, Snippet = {SnipID, _, _}, Expected, _Plugins) ->
                     _ ->
                         generate_config(ConfFile, AdvancedFile)
                 end,
-    Gen = deepsort(Generated),
-    Exp = deepsort(Expected),
+    {Exp, Gen} = case Sort of
+                     true ->
+                         {deepsort(Expected), deepsort(Generated)};
+                     false ->
+                         {Expected, Generated}
+                 end,
     case Exp of
         Gen -> ok;
         _         ->