If a cloud you've previously authenticated with goes away -- as ours
sort of did, because the cloud endpoing in the CLI changed (to actually
be correct) -- then you can't logout without manually editing the
credentials file in your workspace. This is a little annoying. So,
rather than that, let's have a `pulumi logout --all` command that just
logs out of all clouds you are presently authenticated with.
d89a2b4e1f