Add option --skip-mysql or --system-only for pt-stalk

Currently, pt-stalk supports the option --mysql-only that allows skipping system data collection. But sometimes we need the opposite option: --system-only or --skip-mysql to collect only operating system data. Otherwise, if the tool is called, for example, on a host that is running PostgreSQL server, it refuses to start. Workaround is to install minimal MySQL server that is weird.