mysql-sandbox should auto-detect tokudb engine

[abg]

This can be similar to the logic of enabling the innodb plugin for 5.1. Namely we want to set plugin-load = ha_tokudb, if necessary. This is usually done during the bootstrap phase (where share/mysql_system_tables_data.sql injects appropriate rows into mysql.plugin), but depending on the toku deployment the plugin may be loaded via other mechanisms. dbsake currently skips the data load stage for an existing data dir - so bootstrapping from an existing backup can be a little strange in this regard. If that datadir doesn't have mysql.plugin seeded with tokudb this needs to be enabled via config.