command/login: Show login consent prompt before proceeding
command/login: Show login consent prompt before proceeding

Because we're going to pass the credentials we obtain on to some
credentials store (either a credentials helper or a local file on disk)
we ought to disclose that first and give the user a chance to cancel out
and set up a different credentials storage mechanism first if desired.

This also includes the very beginnings of support for the owner password
grant type when running against app.terraform.io. This will be used only
temporarily at initial release to allow a faster initial release without
blocking on implementation of a full OAuth flow in Terraform Cloud.

apparentlymart/terraform,VladRassokhin/terraform,pradeepbhadani/terraform,apparentlymart/terraform,pdecat/terraform,pryorda/terraform,tommynsong/terraform,heimweh/terraform,heimweh/terraform,VladRassokhin/terraform,hashicorp/terraform,tommynsong/terraform,gechr/terraform,pryorda/terraform,pdecat/terraform,nlamirault/terraform,pradeepbhadani/terraform,gechr/terraform,rnaveiras/terraform,ewbankkit/terraform,heimweh/terraform,apparentlymart/terraform,heimweh/terraform,rnaveiras/terraform,apparentlymart/terraform,VladRassokhin/terraform,rnaveiras/terraform,VladRassokhin/terraform,nlamirault/terraform,apparentlymart/terraform,pryorda/terraform,nlamirault/terraform,tommynsong/terraform,heimweh/terraform,ewbankkit/terraform,gechr/terraform,hashicorp/terraform,pdecat/terraform,ewbankkit/terraform,pdecat/terraform,ewbankkit/terraform,pradeepbhadani/terraform,hashicorp/terraform