When initializing a backend, if the currently selected workspace does
not exist, the user is prompted to select from the list of workspaces
the backend provides.
Instead, we should automatically select the only workspace available
_if_ that's all that's there.
Although with being a nice bit of polish, this enables future
improvments with Terraform Cloud in potentially removing the implicit
depenency on always using the 'default' workspace when the current
configuration is mapped to a single TFC workspace.
Chris Arcand