Fix debusine-admin command to create workspaces

In #527 (closed) and !1434 (merged) we changed the create_workspace command to be able to specify and/or create an owner group. In !1434 (comment 555791) I tried to suggest that the group name should be a parameter that the user has to provide because there's no good default. Right now if you don't pass --with-owners-group you get a new group named Owners-<workspace>, that's not really desirable. However forcing the user to pass the group name breaks backwards compatibility for this command.

So my suggestion is to introduce debusine-admin workspace create <scope>/<workspace> <group>, i.e. a new command following the new layout already used for the debusine-admin group <action> set of commands. Since it's a new command, it's not a big deal to break backwards compatibility. The indicated group needs to already exist, otherwise it's a failure.

While we are at it, we should also fix the default value of --default-expiration-delay to 30. Keeping everything for eternity is not a good default in the context of debusine. And we should document in the help text that "0" means "no expiration at all".

/cc @beuc