Add support for catalogs to "ignore_db_dirs"

sql_show.cc:ignore_db_dirs_append is used by engines like rocks_db that has their own directory to store their data.
In a non-catalog server it is used to ensure that these directories are not shown in 'show databases'.
In a catalog-server this has to be changed so that we instead ignore catalogs in
init_catalog_directories (or if this is already been run, delete the catalog entry from catalog_hash)