Split `cnpg` componentsDefinition

[vicentinileonardo]

Currently it is:

  cnpg:
    description: "CloudNativePG operator, PostgreSQL cluster, and database"
    steps:
      - install-cnpg
      - install-pg-cluster

To support the use-case: "CNPG already used in the same K8s cluster" and so we need to skip only the install-cnpg step but keep install-pg-cluster.