Use previous reward address when sending node transaction

[Neylix]

Describe the problem you discovered

When a node bootstrap, it may send a new node transaction. But if the node was already known by the network and its reward has been changed, the bootstrap node transaction send the one by default (env variable or the address of the node chain).
It should take the last reward address known by the network.

Describe the solution you'd like

When the node bootstrap (module Bootstrap) before sending the new node transaction (in first_initialization and update_node function) it should get the last reward address of the node if it exists