user-job-throttle

the number of gwtoolsetUploadMediafileJobs allowed to be added by a
gwtoolsetUploadMetadataJob was throttled by a value the user had no
control over. while testing the extension we realized that this value
is too generic and that the end user should have some control over
its setting.

Bug: 58417
Change-Id: I8fc0850bdf7b65b9e7ef2dd41c1b96901fd91828