info: rename box.info.cluster -> replicaset

It was named 'cluster', but really was just about the replicaset.
This is going to be even more confusing soon, because there will
be introduced an actual concept of cluster as multiple
replicasets.

The patch renames it to 'replicaset'. `box.info.cluster` now means
the whole cluster and is empty so far. Next patches will add here
the cluster name.

Part of #5029

@TarantoolBot document
Title: `box.info.cluster` is renamed to `box.info.replicaset`

Done since 3.0.0. The old behaviour can be reverted back via the
`compat` option `box_info_cluster_meaning`.

`box.info.cluster` key is still here, but now means a totally
different thing - the entire cluster with all its replicasets.

<h2>Compat documentation</h2>

`box.info.cluster` default meaning is the whole cluster with all
its replicasets. To get info about only the current replicaset
`box.info.replicaset` should be used.

In old versions (< 3.0.0) `box.info.cluster` meant the current
replicaset and `box.info.replicaset` didn't exist.

<h3>Old and new behaviour</h3>

New behaviour:
```
tarantool> box.info.cluster
---
- <some cluster keys>
...

tarantool> box.info.replicaset
---
- uuid: <replicaset uuid>
- <... other attributes of the replicaset>
...
```
Old behaviour:
```
tarantool> box.info.cluster
---
- uuid: <replicaset uuid>
- <... other attributes of the replicaset>
...

tarantool> box.info.replicaset (= nil on < 3.0.0)
---
- uuid: <replicaset uuid>
- <... other attributes of the replicaset>
...
```

<h3>Known compatibility issues</h3>

VShard versions < 0.1.24 do not support the new behaviour.

<h3>Detecting issues in you codebase</h3>

Look for all usages of `box.info.cluster`, `info.cluster`, and
even just `.cluster`, `['cluster']`, `["cluster"]`. For the new
behaviour to work all of them have to use 'replicaset' key.