Add configuration option to disable start-up database schema check

Currently jBPM 4 checks whether the system tables are present in the database during the initialization phase. Sometimes this can cause issues as described in the following forum entry: http://community.jboss.org/message/567453

Please add some kind of configuration option to enable/disable start-up database schema checking. This behavior can be disabled by calling org.jbpm.pvm.internal.cfg.ConfigurationImpl.skipDbCheck() from Java code but there is no end user access for this: there is no way to change this setting from any configuration file.