Start uwsgi in the 'emperor' mode, convert old configs

Emperor mode is the suggested way to start system-wide uWSGI service.
It will start separate uWSGI instance for every available config file.
Instance configuration files are placed in /etc/uwsgi/vassals

Old ini or xml configuration file will be converted and placed in the
vassals directory on upgrade from older package. The convertion is not
fail-proof, though.
systemd support added