This change implements some feedback from @ellismg.
* Make backend.Stack an interface and let backends implement it,
enabling dynamic type testing/casting to access information
specific to that backend. For instance, the cloud.Stack conveys
the cloud URL, org name, and PPC name, for each stack.
* Similarly expose specialized backend.Backend interfaces,
local.Backend and cloud.Backend, to convey specific information.
* Redo a bunch of the commands in terms of these.
* Keeping with this theme, turn the CreateStack options into an
opaque interface{}, and let the specific backends expose their
own structures with their own settings (like PPC name in cloud).
* Show both the org and PPC names in the cloud column printed in
the stack ls command, in addition to the Pulumi Cloud URL.
Unrelated, but useful:
* Special case the 401 HTTP response and make a friendly error,
to tell the developer they must use `pulumi login`. This is
better than tossing raw "401: Unauthorized" errors in their face.
* Change the "Updating stack '..' in the Pulumi Cloud" message to
use the correct action verb ("Previewing", "Destroying", etc).
2eb86b24c2