Clarify configuration and job definition

The docs was a little confusing in that used a single term 'configuration' to
referred to both the JJB global configuration and the job definitions.
This change clarifies that by changing 'configuration' in a few places
to 'job definition' to make it less confusing.

Change-Id: Ie64e7d899b793c9c8e86e51076c020640e336a40
Merge "Clarify configuration and job definition"