raft: introduce box.cfg.election_* options

The new options are:

- election_is_enabled - enable/disable leader election (via
  Raft). When disabled, the node is supposed to work like if Raft
  does not exist. Like earlier;

- election_is_candidate - a flag whether the instance can try to
  become a leader. Note, it can vote for other nodes regardless
  of value of this option;

- election_timeout - how long need to wait until election end, in
  seconds.

The options don't do anything now. They are added separately in
order to keep such mundane changes from the main Raft commit, to
simplify its review.

Option names don't mention 'Raft' on purpose, because
- Not all users know what is Raft, so they may not even know it
  is related to leader election;
- In future the algorithm may change from Raft to something else,
  so better not to depend on it too much in the public API.

Part of #1146