fix(ynh_setup): change user first name / last name to full name

BREAKING CHANGE: With Yunohost 11.1 onwards, Yunhost users must be created with full name only. This breaks YunoHost postinstall and user creation because with Ansible we now have such errors:

[
    "Not a tty, can't do interactive prompts"
]

This is because YunoHost waits for a full name to be given but can't because our process is automated and not interactive.

This issue is raised:

• https://forum.yunohost.org/t/yunohost-11-1-release-sortie-de-yunohost-11-1/23378#notes-for-advanced-users-and-packagers-7
• https://forum.yunohost.org/t/yunohost-11-1-release-sortie-de-yunohost-11-1/23378#gift-detailed-changelog-8