Bootstrap in RO is allowed in the supervised bootstrap strategy

[TarantoolBot]

Now, it is OK to configure all the instances of a replicaset in
read-only (box.cfg({read_only = true})) and issue
box.ctl.make_bootstrap_leader(<...>) from an external agent on one of
the instances.
Requested by @Totktonada in tarantool/tarantool@54ea299.