Config.hs: Add a function to lookup group

Like in the python code, this is a bit more complex since groups are
indexed by UUID, so we must fallback to lookup via the name (slow; but
the number of groups should be small).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>