Add description to workspace variables

[stasquatch]

Description

Workspace variables (terraform and env) now support descriptions. These descriptions are complete separate from descriptions defined in an ingressed config, but appear in the Terraform Cloud UI where Variables are configured.

Testing plan

Create a variable with a description like:

resource "tfe_variable" "pet_name_length" {
    key = "pet_name_length"
    value = "5"
    description = "length of pet name in words"
    category = "terraform"
    workspace_id = tfe_workspace.workspace_name.id
}

Local test results 02/10/2020

[lafentres]

updating isn't working (adding description to variable that doesn't have one, changing description, etc). i think it needs to be added to the resourceTFEVariableUpdate method.

i also ran into some errors running make build because go fmt needs to be run on tfe/resource_tfe_variable.go.

[lafentres]

🎉🎉🎉🎉