Remove unescaped WORKSPACE vars in JJB config.

* .../jenkins_job_builder/config/jenkins-plugin-jobs.yaml: An unescaped
WORKSPACE variable was introduced to the JJB config that resulted in
a JJB KeyError. Remove these unescaped WORKSPACE vars as they are not
necessary.

Change-Id: I68417079d5633d3bf9e2f3449fc3285ac800f595
Reviewed-on: https://review.openstack.org/36141
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins