plainbox: make setup.py work in any directory

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
support: add specially-crafted setup.py for rtfd.org

This setup.py file is discovered by rtfd.org build and proceeds to
multiplex setup.py commands to both plainbox and checkbox-ng

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
support: add rtfd.org requirement file

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
support: trigger rtfd.org rebuild after testing

This acts as a poor-man's post-land hook that we invoke from our tarmac
build / test machine. It causes a rebuild to be triggered on rtfd.org

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>