Fix issue 944 - allow org users to read shared catalogs

[dataclouder]

Address issue #944

• Fix title of catalog operations guide

• Add functions to find a catalog without its Org

  These changes fix the problem of a catalog that was shared
  to the current org, but a tenant could not retrieve it
  for lack of privileges.
  It also fixes the case when multiple catalogs with the same
  name are available in the organization.
  The new functions search for catalogs without need of accessing
  the sharing organization.
  Also added a new 'is_local' property to all catalog resources and data sources.
  This property tells whether the catalog was created in the current org
  or not.

When there are multiple catalogs with the same name in one Org, and users attempt retrieving a data source, there are two possibilities:

1. They know the name of the sharing organization, from where the catalog comes. So they can use that name in the org field. The data source gets retrieved as if it were local.
2. They don't know the original organization name. So they attempt to retrieve the data source, and get an error message that will help retrieving the right one:

│ Error: [catalog read DS] error retrieving catalog cat-datacloud: no records found for catalog 'cat-datacloud' from org 'datacloud-1'
│ There are catalogs with the same name from other orgs: [datacloud datacloud-2]
│
│   with data.vcd_catalog.catalog-1,
│   on config.tf line 32, in data "vcd_catalog" "catalog-1":
│   32: data "vcd_catalog" "catalog-1" {
│

Side effect: There is still a problem, which existed before, but has been made official by an appropriate error message in this PR: when searching a data source by filter (not by name), the filter only works if the Org containing the catalog is reachable by the user. Thus, it won't work for a shared catalog.