Fix logic to create/delete/update sensitive variables

[alfespa17]

Fix internal logic when creating resources with "sensitive=true" like the following:

resource "terrakube_organization_variable" "sample0" {
  organization_id = data.terrakube_organization.org.id
  key             = "org-sensitive"
  value           = "sensitive"
  description     = "sample"
  category        = "ENV"
  sensitive       = true
  hcl             = false
}

resource "terrakube_workspace_variable" "sample1" {
  organization_id = data.terrakube_organization.org.id
  workspace_id    = terrakube_workspace_cli.sample1.id
  key             = "workspace-sensitive"
  value           = "sensitive"
  description     = "Sample"
  category        = "ENV"
  sensitive       = true
  hcl             = false
}

Fix #36