Allow migration from gdm to gdm-container in one go

[kalikiana]

As a user of MicroOS with GNOME desktop I need to remove gdm (and patterns-microos-desktop-gnome). gdm-container will refuse to work before the new snapshot is active meaning I need to reboot without a GUI. I assume it could work in one go except for the package checks currently in place.

• Maybe the state of the pending snapshot can be used for reference?
• Prompt to prepare the snapshot with the right dependencies, which then is known to work after a reboot.
• If all else fails, an argument or variable to set, to skip the dependency check.