decouple from workspace ids

[davcen]

The current configuration of this module define an implicit dependency between the variable set and the workspaces that use it, through the variable workspace_ids (and relevant resource tfe_workspace_variable_set)

This logic must be reversed, in order to define an indipendent variable set resource (as i should be, because i can create a variable set in Terraform Cloud UI without assign it to any specific workspace)

A specular change must be applied in https://github.com/flowingis/terraform-tfe-workspace