vshard: `box_cfg_mode` cfg option on router

[TarantoolBot]

Related dev. issue: n/a

Product: Tarantool, vshard
Since: 3.0, 0.1.25
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_rock/vshard/vshard_router/
SME: @ Totktonada, @ Serpentian

Details

The option can be specified at the root level and regulates, whether
vshard calls box.cfg, when vshard.router.cfg() is invoked.

When specified to 'auto' (default), vshard configures box.cfg on its
own. When specified to 'manual', vshard's router can be started even
with unconfigured box.cfg.
Requested by @Serpentian in tarantool/vshard@7a8253c.