tt cluster publish can publish a new instance configuration

[TarantoolBot]

Product: tt CLI
Since: 2.3.0
Root document: https://www.tarantool.io/en/doc/latest/reference/tooling/tt_cli/cluster/
SME: @ askalt

Details

This patchs adds an ability to publish a new instance configuration with tt cluster publish.
Prior this patch this command returns an error if the instance is not found in the config.

Now, to create a new instance configuration, a replicaset name with --replicaset can be specified,
and, if it's not enough, a group name with --group.

For example:

$ tt cluster show http://localhost:2379/foo
/foo/config/all
credentials:
  users:
    guest:
      roles:
        - super
groups:
  group-001:
    replicasets:
      replicaset-001:
        instances:
          instance-001:
            iproto:
              listen:
                - uri: localhost:3301
          instance-002:
            iproto:
              listen: {}

$ tt cluster publish http://localhost:2379/foo?name=instance-003 inst.yml
   ⨯ failed to create an instance "instance-003" configuration: replicaset name is not specified

$ tt cluster publish --replicaset replicaset-001  http://localhost:2379/foo?name=instance-003 inst.yml

$ tt cluster show http://localhost:2379/foo
credentials:
  users:
    guest:
      roles:
        - super
groups:
  group-001:
    replicasets:
      replicaset-001:
        instances:
          instance-001:
            iproto:
              listen:
                - uri: localhost:3301
          instance-002:
            iproto:
              listen: {}
          instance-003:
            database:
              mode: rw

Part of #316
Requested by @askalt in tarantool/tt@efe95f1.