catalog: put handler plugins configuration under "handler" key

Before:

   {
     "datasets": {
       "DATASET": {
         "plugins": ["serach", "groonga", "add"]
       }
     }
   }

After:

   {
     "datasets": {
       "DATASET": {
         "handler": {
           "plugins": ["serach", "groonga", "add"]
         }
       }
     }
   }

The old syntax is old available.