Add support for global variables

[alfespa17]

Adding support for organization_variables.

resource "terrakube_organization_variable" "sample1" {
  organization_id = data.terrakube_organization.org.id
  key             = "sample-env-var"
  value           = "sample-var2"
  description     = "sample env var2213241243"
  category        = "ENV"
  sensitive       = false
  hcl             = false
}

Note: once created the API does not allow to change the sensitive field value