Auto-delete PVCs

[dan-fernandes]

Since 8cd830 BlueAPI can retain PVCs. This means that a BlueAPI deployment will use up cluster space indefinitely, if these PVCs are not manually deleted by an engineer.

Having discussed with @DominicOram and @noemifrisina the deletion of PVs should be automated, with the following requirements:

1. The three most recent PVCs are retained
2. PVCs can be tagged with something to stop their deletion
3. PVC deletion should be loud (emails, slack, etc. with fair time beforehand)

[keithralphs]

Dom also mentioned today that it would be good if, on marking for deletion, if a PVC had changes to the code in any of the repos checked out to it, there was the option to create a branch in that repo to which the changes would be pushed in order to be dealt with or actualy discarded. This raises the question of 'pushed as which user', if this option were part of the notification of intention to deleet the PVC, then presumably one of the users so notified, who would be taking ownership for whether the deletion should go ahead, could be the user as whom the push to github was made.