Job-specific subst. in a job group's job list

Adding job-specific substitutions in a job group's job list
would result in JJB crashing with the following error:
  TypeError: unhashable type: 'dict'

This change adds the ability to add job-specific substitutions
in a job group's job list. It implements the same logic used by
a previous change (6ff1d69).

Change-Id: I059f38d0aa8a8947aaac75e0ddb34d392063ece3
Merge "Job-specific subst. in a job group's job list"