Make only the first node in group0 to start as voter. Subsequent nodes
start as non-voters and request change to voter once bootstrap is
successful.
Add support for this in raft and a couple of minor fixes.
* alejo/raft-join-non-voting-v6:
raft: nodes joining as non-voters
raft: group 0: use cfg.contains() for config check
raft: modify_config: support voting state change
raft: minor: fix log format string
7ae947b7e1