Confd IPv6 support

This patch series basically adds a new parameter 'family' to the constructors
of daemon.AsyncUDPSocket and confd.client.ConfdUDPClient. This enables the
users of these two classes to support IPv6.

In ganeti-confd.ConfdAsyncUDPClient a method to check the address families of
all peers is added.

Furthermore it adds unittests for the added functionality.

Signed-off-by: Manuel Franceschini <livewire@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>