fix(cass): avoid using aggregates in healthcheck query

Otherwise, this query will produce warning message per each
healthcheck, telling the user:
```
WARN  <...> - Aggregation query used without partition key
```

apps/emqx_bridge_cassandra/src/emqx_bridge_cassandra.app.src

@@ -1,6 +1,6 @@
 {application, emqx_bridge_cassandra, [
     {description, "EMQX Enterprise Cassandra Bridge"},
-    {vsn, "0.1.4"},
+    {vsn, "0.1.5"},
     {registered, []},
     {applications, [
         kernel,

apps/emqx_bridge_cassandra/src/emqx_bridge_cassandra_connector.erl

@@ -332,7 +332,7 @@ on_get_status(_InstId, #{pool_name := PoolName} = State) ->
     end.
 
 do_get_status(Conn) ->
-    ok == element(1, ecql:query(Conn, "SELECT count(1) AS T FROM system.local")).
+    ok == element(1, ecql:query(Conn, "SELECT cluster_name FROM system.local")).
 
 do_check_prepares(#{prepare_cql := Prepares}) when is_map(Prepares) ->
     ok;