schema: replace 'cluster' -> 'replicaset_uuid'

Replicaset UUID was stored in _schema['cluster'] tuple. This is
going to be confusing soon, because there will be introduced an
actual concept of cluster as multiple replicasets.

The patch renames it to 'replicaset_uuid'.

Part of #5029

@TarantoolBot document
Title: Update '_schema' with new 'replicaset_uuid' key

Currently _schema system space is documented to have 'cluster' key
with replicaset UUID value. Now this key is deleted (since 3.0)
and the UUID is stored in 'replicaset_uuid' key.