Remove use_default_creds from preprov creds

The preprovisioned credentials provider used to work as a proxy
to legacy (default) credentials via the legacy credentials provider.
Now that legacy credentials have been removed along their provider,
the use_default_creds flag doesn't make sense anymore.
The credential provider should just raise an exception on __init__
if no account file is specified.

Change-Id: Iedffea5493f650eb59ec240910f718a16fd680cd
1 file changed