Default admin on app install

[zamentur]

When we install an app that need an admin, we sometimes have just one user OR we have only one admin listed in admins group. In this case, it could be useful to prefill this question with this user (and let the instance admin change the answer if needed).

Example of what's happened currently

root@ynh:/ynh-dev# yunohost app install grafana
[...]
Choose an administrator user for this app [toto]:
Error: Argument 'admin' is required

And what it could be:

root@ynh:/ynh-dev# yunohost app install grafana
[...]
Choose an administrator user for this app [toto]: toto <- prefilled answer

It might be managed here (i am not sure): https://github.com/YunoHost/yunohost/blob/069b782f07838ff42e96361620700bc7749ee15d/src/utils/config.py#L1319

[alexAubin]

Yup in fact there's a FIXME a few lines below: https://github.com/YunoHost/yunohost/blob/069b782f07838ff42e96361620700bc7749ee15d/src/utils/config.py#L1343