Made db storage engine configurable

These changes make the choice of the mysql storage engine a parameter configurable in site.conf/localOverrides.conf.  Default setting is 'myisam' for both the webwork tables and the opl tables.  However, this can be changed my modifying $database_storage_engine or $problemLibrary_db{storage_engine}.  Also modified OPL-update to use db credentials from the $problemLibrary_db hash. Conceivably someone might want to expose the problem library to a separate web application as well as webwork and so not want the same db credentials. This will probably have to be fleshed out more to make it possible, but the $problemLibrary_db hash was already in defaults.config and not being used by OPL-update.