Common group sylva for all OSs

Actually the default and user injected cloud-init users must be part of a group to remotely connect to the OS via SSH. Depending on the OS the group is different:

• Ubuntu => sudo
• SuSE => users

We could declare a group named sylva on both OSs:

• Same group on both platform ease the configuration for all OS like SSH configuration for example.
• Does not use builtin groups which could have some privileges like vanilla sudo
• Ease the user configuration and documentation

We have to be careful this new group is correctly used by default cloud-init user as well our configuration files use it properly.

WARNING: It could have a huge impact on deployed platform. We have to document and reflect this change in sylva-core