vshard: `box_cfg_mode` cfg option

[TarantoolBot]

Related dev. issue(s): tarantool/vshard#428
Related doc. issue(s): #3895

Product: Tarantool, vshard
Since: 3.0
Root document: https://docs.d.tarantool.io/en/doc/3.0/reference/reference_rock/vshard/vshard_ref/
SME: @ Serpentian

Details

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

When specified to 'auto' (default), vshard configures box.cfg on its
own. When specified to 'manual', it's user's responsibility to call
box.cfg prior to vshard's configuration
Requested by @ Serpentian in tarantool/vshard@95325a9.