LUClusterVerifyGroup: Spread SSH checks over more nodes

When verifying a group the code would always check SSH to all nodes in
the same group, as well as the first node for every other group. On big
clusters this can cause issues since many nodes will try to connect to
the first node of another group at the same time. This patch changes the
algorithm to choose a different node every time.

A unittest for the selection algorithm is included.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>