bootstrap_leader only makes sense on initial node bootstrap and doesn't have any effect further on, so the option was made static (one can't change it after initial box.cfg call).

This leads to a situation when each node has its own box.cfg.bootstrap_leader setting (depending on which node was master back then).

However for centralized configuration it's much more handy to have box.cfg options in sync between all the instances of a replica set.

Let's make the bootstrap_leader option dynamic, even though it won't take any effect when changed after initial box.cfg or even on initial box.cfg, if bootstrap is already perfomed.