The Cluster object, as it is defined right now, has many '[String]'
members, which means that in a standard arbitrary generator these will
become very big, which is the reason for the current slowness of the
test 'Config_serialisation'.

By resizing the generator to 8 (arbitrary chosen to limit the list
length and the string sizes), and by reducing a bit the node count, we
can make this test be as fast as the others (about 10x
improvement). This means we can test more cases, for the same cost.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>