Implement pod cleanup cronjobs

[shreddedbacon]

This adds 2 cronjobs to the manager

• Build pod cleanup
  • Runs every hour on the hour
  • Retains 1 pod per namespace that isn't running/pending
• Task pod cleanup
  • Runs every hour on the half hour
  • Retains 1 pod per namespace that isn't running/pending

These are all configurable with flags

#builds defaults
--enable-build-pod-cleanup=true
--build-pod-cleanup-cron="0 * * * *"
--num-build-pods-to-keep=1
#tasks defaults
--enable-task-pod-cleanup=true 
--task-pod-cleanup-cron="30 * * * *"
--num-task-pods-to-keep=1

closes #39