vshard: config identification mode

Related dev. issue(s): https://github.com/tarantool/vshard/issues/426

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

Details

The option identification_mode should be specified in the root of the
config. It can have one of those values:

• 'uuid_as_key' - default. Means, that default uuid config
  identification is used. replica.name is allowed and should not be
  interpreted as box.cfg.instance_name. replica/replicaset.uuid is
  forbidden. The config should have the following format:

  {
      ['cbf06940-0790-498b-948d-042b62cf3d29'] = { -- replicaset #1
          replicas = {
              ['8a274925-a26d-47fc-9e1b-af88ce939412'] = {
                  name = 'storage_1_a',
                  ...
              },
              ...
          },
      },
      ...
  }
  

• 'name_as_key'. Name identification is used, supported only by
  Tarantool >= 3.0.0. It's forbidden to specify replica.name in
  such format. UUIDs are optional and can be specified via
  replicaset/replica.uuid:

  {
      replicaset_1 = {
          uuid = 'cbf06940-0790-498b-948d-042b62cf3d29',
          replicas = {
              replica_1_a = {
                  uuid = '8a274925-a26d-47fc-9e1b-af88ce939412'
                  ...
              },
              ...
          }
      },
      ...
  }
  

  Note, that names, used as keys in config are passed to
  box.cfg.replicaset/instance_name for storage. In case of
  reconfiguration it's strictly validated, that both
  replicaset and instance name corresponds to the passed
  config. Vshard doesn't deal with changing or setting names,
  it must be done externally (using Tarantool's config module,
  for example).
  Requested by @ Serpentian in https://github.com/tarantool/vshard/commit/1f8d7fb6a74f5c84567c11de2c558739bb6e3bd6.

Start with the vshard reference documentation at the linked root document and locate the configuration section. Document the root-level identification_mode values, their UUID/name key formats, validation rules, and Tarantool version restriction using the examples in this issue; done means the reference explains both supported modes accurately.