Filter credentials when returning job binaries through REST api

The 'extra' field for job binaries is used to store credentials
for accessing job binaries. Modify the JobBinary resource to
specify 'extra' as a filtered field.

Closes-bug: #1272322
Change-Id: I92c01bd8015e6e515081f83faa186de004a35a39
Merge "Filter credentials when returning job binaries through REST api"