It seems that ynh takes care itself of registering the app on restore.
So re-registering it is useless and triggers a duplicate error. Let's not do
that :-).
Instead of doing it by hand.
Also we now rely on the fact ynh automatically runs uninstall script if the
install fails. So no more need for manual cleanup.
Huge change in package, mainly because we switch from a tarball-based install to a
pip-based install (which is the default install method).
It require quite some refactor to common.sh and a more consistent and clear
migration handing in upgrade script.
Fix#19#16
