MDEV-12914: Engine for temporary tables which are implicitly created as RocksDB is substituted silently

- There should be no substitution if engine exists, only when doesn't
exist
- Handling of an error when sys_var `default_tmp_storage_engine` is
assigned to unsupported engine.
- rocksdb doesn't support embedded server ebfc4e6a so is excluded

Closes PR #774
Reviewed by: serg@mariadb.com
             vicentiu@mariadb.org