sam deploy guided doesn't allow specifying bucket

### Description

A new stack is created instead of updating an existing stack when using `--guided`.

`--guided` does not allow me to specify the bucket

### Steps to reproduce

Create a stack without a `samconfig.toml`

Attempt to update the stack using `sam deploy --guided`

### Observed result

A new stack is created / the name of bucket is never asked for.

### Expected result

Bucket name is asked for, existing stack is modified.

### Additional environment details (Ex: Windows, Mac, Amazon Linux etc)

1. OS: osx 10.15.1
2. `sam --version`: SAM CLI, version 0.39.0

`Add --debug flag to command you are running`