Improve debusine-admin group subcommands; allow listing existing groups

• The debusine-admin group subcommands are in need for slight improvement. While there is a mechanism to create groups, listing existing groups would be a really useful feature. The current subcommands are create, rename, delete and members. Notably missing is list and omitting the action is not supported.
• Then debusine-admin group members spam could use a nicer error message. It really expects a scoped group name (with a slash here) and all you get is traceback ending in a ValueError. While it is easy to understand what should be done, this likely is not worth a traceback.
• For some reason, I am also unable to extract a member listing debusine-admin group members $existing_scope/$existing_group_with_members does not yield any output and exits successfully. I double checked that the named group exists in the named scope and has members.