Unclear how to refresh a user session for HCP provider

[sbussetti]

I filed this originally on the generated repo:
hashicorp/terraform-provider-hcp#450

basically the sso browser dance works one time from terraform if I provide no parameters to the hcp provider. once that expires and I try to apply terraform a second time, it just tells me i have no valid auth and does not retrigger the browser based login flow with HCP.