fix variable name collision in discovery recv_neighbours

[janx]

Problem

In recv_neighbours of discovery.py, variable node is reused to get neighbours set, make node refer to the last neighbour instead of the packet sender.

Solution

Use a different variable _node to get neighbours.

[coveralls]

Coverage decreased (-0.3%) to 87.856% when pulling 918672e on janx:fix_recv_neighbours into 3121147 on ethereum:develop.