box.cfg.bootstrap_leader accepts instance names now

[TarantoolBot]

The option box.cfg.bootstrap_leader, which specifies the desired
bootstrap leader when bootstrap_strategy is "config" now accepts
instance names.

For example, this is a valid config without replication:

box.cfg{
    instance_name = 'main-server',
    bootstrap_strategy = 'config',
    bootstrap_leader = 'main-server'
}

When box.cfg contains some entries in replication, the node will
bootstrap from the node which has the instance name specified in
box.cfg.bootstrap_leader.

This is an addition to #3432
Requested by @sergepetrenko in tarantool/tarantool@8ceef02.

[andreyaksenov]

https://docs.d.tarantool.io/en/doc/3.0/reference/configuration/configuration_reference/#configuration-reference-replicasets-name-bootstrap-leader