Fix job-specific parameters for job groups.

When a job template is instantiated via a job-group,
job-specific parameters provided by the project are not
respected.

The correction is just a matter of updating correctly the
dictionary containing all parameters.

The configuration below exhibits the issue:

- project:
    name: project-with-specific-parameters
    jobs:
        - my-job-group:
            recipients: "foo@example.com"

- job-group:
    name: my-job-group
    jobs:
        - job-sending-notification-mails

- job-template:
    name: job-sending-notification-mails
    publishers:
      - email-ext:
          # {recipients} should be available.
          recipients: "{recipients}"
          subject: Subject for Build ${{BUILD_NUMBER}}


Change-Id: I107d8d648c8965cd125db52f3558f28337c25a63
Reviewed-on: https://review.openstack.org/33190
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins