Add a NEXUS parameter to define its address

Add a NEXUS parameter to define its address within the job "generate-zip-files.groovy" [1]
in order to avoid to hard code here [2][3]

[1] https://gitlab.cee.redhat.com/middleware/productization-jenkins-pipeline-jobs/blob/master/prod/generate-zip-files-job.yaml
[2] https://gitlab.cee.redhat.com/middleware/productization-jenkins-pipeline-jobs/blob/master/prod/generate-zip-files.groovy#L59
[3] https://gitlab.cee.redhat.com/middleware/productization-jenkins-pipeline-jobs/blob/master/prod/generate-zip-files.groovy#L82