IFP: add org.freedesktop.sssd.infopipe.Groups.Group
    
    Example calls:
    
    dbus-send --print-reply --system --dest=org.freedesktop.sssd.infopipe /org/freedesktop/sssd/infopipe/Groups/LDAP_2ePB/30001 org.freedesktop.DBus.Properties.Get string:org.freedesktop.sssd.infopipe.Groups.Group string:name
    method return sender=:1.159 -> dest=:1.168 reply_serial=2
       variant       string "group-1"
    
    dbus-send --print-reply --system --dest=org.freedesktop.sssd.infopipe /org/freedesktop/sssd/infopipe/Groups/LDAP/30002 org.freedesktop.sssd.infopipe.Groups.Group.UpdateMemberList
    method return sender=:1.934 -> dest=:1.937 reply_serial=2
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2150
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>