Filter credentials in jobs returned from REST api

The 'mains' and 'libs' elements of a Job contain references
to JobBinary objects, which can contain credentials in the
'extra' field.  Filter 'extra' from 'mains' and 'libs' when
returning a wrapped Job object.

Closes-Bug: #1272452
Change-Id: Ieccadbbda1f081595374469cb938b7bffbc08e17
Merge "Filter credentials in jobs returned from REST api"