multistore_file.py: don't write trailing whitespace in json creds

I've noticed this minor infelicity in 'google-cloud-sdk'
project which creates prettyprinted cred files in

    ~/.config/gcloud
    ~/.config/gcloud/legacy_credentials/${USER}/multistore.json

with trailing whitespace.

The reason is the call to 'json.dump[s](indent=2)'
without accompanying 'separators=' overrride.

Signed-off-by: Sergei Trofimovich <siarheit@google.com>