Move by default to python3 and systemd with many fixes
- keep burpui-agent to 0.6.1 as: can't migrate to 0.6.5 until upgrade burp to 2.2.12+
- Update python3 to python3.6 on Redhat/Centos
- Move default installation to python3
- Support bui-celery to systemd
- Move default to systemd