Retry fetch_file (cpi).
Retry fetch_file (cpi).

We already have a retry for the upload path.
This change adds the retry for the download path.

Test:
-bosh deploy/delete
-spec tests

Change-Id: I41c3b2e7c3e9707d39cd1b9a7ed63eb547a0ba89

barthy1/bosh,barthy1/bosh,barthy1/bosh,barthy1/bosh