solana-fullnode: Consider making -l mandatory

[mvines]

Currently if -l is not provided on the command-line, a default is supplied:

solana/src/bin/fullnode.rs

Line 111 in 7fe1fd2

let mut repl_data = ReplicatedData::new_leader(&bind_addr);

Is this really the behaviour we want?

[garious]

The default allows you to do single node testing without the extra step of generating a config file first. Just trying to let simple things be simple. Did the default trip you up in the multinode workflow?