garbage collection worker to clean-up hanging jobs

[senthilrch]

if the imagepuller pod is unable to pull the image, it keeps retrying. The job that created the imagepuller pod has an activeDeadlineSeconds = 3600. so users can query the imagepuller pod's events to understand the reason for failure.

we need a garbage collection worker in image manager to cleanup jobs that have reached activeDeadlineSeconds.

[senthilrch]

Implemented alternate solution. For jobs still active after image-pull-deadline-duration, image manager retrieves the events for the failed/running imagepuller pods and updates it to status of image cache. User can get the details of failure reason directly from image cache status "failures" section. Before updating status update work item to the work queue, image manager deletes all jobs (successful/pending) since they are no longer needed. Refer to following commit:-

654c075