KCC: improve directed_double_ring graph check

KCC: improve directed_double_ring graph check

The previous test assumed there would be only a double directed ring
but in fact there could be other edges.  In large graphs there are
certain to be more edges.

Now we want to be sure there is a complete ring apart from any other
connections. This is called the Hamiltonian path problem and takes
exponential time in general, so now our test is that it looks *quite*
a lot like a complete ring.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>