Hide citus.enable_ddl_propagation setting (#3437)

As that is powerful and cause metadata inconsistency. See the following steps:

(Note that we cannot use PGC_SUSET because on Citus MX we need this flag for non-
superusers as well)

```SQL
CREATE TABLE test_ref_table(key int);
SELECT create_reference_table('test_ref_table');

SELECT logicalrelid, logicalrelid::oid FROM pg_dist_partition;
┌────────────────┬──────────────┐
│  logicalrelid  │ logicalrelid │
├────────────────┼──────────────┤
│ test_ref_table │        16831 │
└────────────────┴──────────────┘
(1 row)

Time: 0.929 ms

SELECT relname FROM pg_class WHERE oid = 16831;
┌────────────────┐
│    relname     │
├────────────────┤
│ test_ref_table │
└────────────────┘
(1 row)

Time: 0.785 ms

SET citus.enable_ddl_propagation TO off;

 DROP TABLE test_ref_table ;

SELECT logicalrelid, logicalrelid::oid FROM pg_dist_partition;
┌──────────────┬──────────────┐
│ logicalrelid │ logicalrelid │
├──────────────┼──────────────┤
│ 16831        │        16831 │
└──────────────┴──────────────┘
(1 row)
Time: 0.972 ms

SELECT relname FROM pg_class WHERE oid = 16831;
┌─────────┐
│ relname │
├─────────┤
└─────────┘
(0 rows)

Time: 0.908 ms

 SELECT master_add_node('localhost', 9703);
server closed the connection unexpectedly
	This probably means the server terminated abnormally
	before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.
Time: 5.028 ms
!>

```

src/backend/distributed/shared_library_init.c

@@ -739,7 +739,7 @@ RegisterCitusConfigVariables(void)
 		&EnableDDLPropagation,
 		true,
 		PGC_USERSET,
-		GUC_STANDARD,
+		GUC_NO_SHOW_ALL,
 		NULL, NULL, NULL);
 
 	DefineCustomBoolVariable(