Datastream already exists error instead of reusnig

## Expected Behavior
I expect the migration tool to reuse the stream if it already exists.

## Actual Behavior
I get error "Already exists". I need to drop and create with new name everytime I need to retry because something failed.

## Steps to Reproduce the Problem

1. Run spanner-migraiton-tool with more than once with the streamid populated in streaming.json file.

## Specifications

- Version: latest
- Platform: unix