Refactor replicas config for Ray Pipelines

[oryx1729]

In the current implementation of RayPipeline, replicas can be configured for each component in the YAML. For instance,

version: '0.9'

components:
  - name: ESRetriever
    type: ElasticsearchRetriever
    replicas: 2   # create replicas
    params:
      document_store: DocumentStore
  - name: DocumentStore
    type: ElasticsearchDocumentStore

pipelines:
  - name: ray_query_pipeline
    type: RayPipeline
    nodes:
      - name: ESRetriever
        inputs: [ Query ]

The replicas is a configuration property for the pipelines rather than of the individual component. This PR updates that and adds a test case. With this, the YAML would look like this:

version: '0.9'

components:
  - name: ESRetriever
    type: ElasticsearchRetriever
    params:
      document_store: DocumentStore
  - name: DocumentStore
    type: ElasticsearchDocumentStore

pipelines:
  - name: ray_query_pipeline
    type: RayPipeline
    nodes:
      - name: ESRetriever
        replicas: 2   # replicas are now configured here
        inputs: [ Query ]

[lalitpagaria]

Can we please update/add documentation as well.

[lalitpagaria]

LGTM!