Add shard group support

[denisb0]

If there's custom shard group defined, operations will break at code like vshard.router.routeall().
It could be fixed by passing shard group name in opts argument and using first group in list if it's not provided/not found.

[Totktonada]

Vshard group is Cartridge's term. Vshard itself only allows to create several vshard router objects. See also:

• [2pt] vshard: Missing info on vshard-groups doc#2450
• [3pt] Никак не описано как работать с vshard-groups в документации cartridge doc#2319

My proposal:

• Add vshard_router parameter (accepts vshard router object).
• Add vshard_group parameter (enabled only if crud-router role is started, accepts a vshard group name).

[Totktonada]

I'm not right: routers are named in vshard (see the documentation). So we can accept router object and name both in the vshard_router parameter.