Initialize 'user' of the OCI driver with the logged in user if not defined

The default behaviour of Oracle is to use the name of the currently logged on user as the schema name. This causes queries for e.g primary key to fail since it requires the full name, including schema, to work. The OCI driver should use this by default if nothing is defined, to allow using the QOCIDriver(OCIEnv* env, OCISvcCtx* ctx, QObject* parent) constructor where existing connections can be specified.