Improve nano::test::system::add_node (...)

[pwojcikdev]

I encountered a problem where nano::test::system::add_node (...) wouldn't properly connect all nodes. I simplified the code, removing legacy bootstrap waits, and it seems to work fine. Additionally I extended the initialization blocks list to

void set_initialization_blocks (std::deque<std::shared_ptr<nano::block>> blocks);
void set_cemented_initialization_blocks (std::deque<std::shared_ptr<nano::block>> blocks);

in order to simplify setting up multiple nodes with the same set of blocks.