Add a feature to Percona Server to allow enforced use of a specific storage engine.

Comsulting # 23310 :
Add a feature to Percona Server to allow enforced use of a specific storage engine.

The server option is --enforce_storage_engine (Ex: --enforce_storage_engine=Innodb) and it may not be changed at runtime.

When this option is specified and a user tries to create a table using an explicit storage engine that is not the specified enforced engine, they will get either an error if the no_engine_substitution SQL_MODE is set or a warinig if no_engine_substitution is OFF and the table will be created anyway using the enforced engine (this is consistent with the default MySQL way of creating as MyISAM if other engines aren't available unless no_engine_substitution is set).