discovery with sql is too restrictive

Currently the SQL syntax that is accepted for sql-based discovery (in TABLE_SHARE::init_from_sql_statement_string() restricts the CREATE TABLE statement to a certain subset of all allowed syntaxes. In particular, one cannot use the ENGINE= clause. This is unnecessary restrictive, one should be able to specify ENGINE=xxx as long as xxx is the engine performing the discovery. Just as before, one should not be able to alter table's engine during the discovery.