Fixed the akquisition of a database-connection, if one is needed

Fixed the akquisition of a database-connection, if one is needed

 * The helper-class SimpleConnectionProvider was not changed to reflect the
   alterd handling of the configuration parameters.
 * If only the user is missing, SimpleConnectionProvider tries to akquire a
   connection, because for example H2 does not always need a user, hence
   this is an sufficient configuration.
 * If an exception is thrown during database akquisition, it is only logged,
   instead of rethrown.
 * Adapted the example-code for the hibernate-bug HHH-9615 to function as
   an additional integration-test and used it, to proof, that an database
   connection can be akquired, if only the driver and the URL is present.