Add a check for cronjobs with a concurrencyPolicy of Allow

[timoreimann]

A cronjob configured with a concurrencyPolicy of Allow has the potential for overloading a cluster if a cronjob-managed pod cannot manage to finish in time. The impact we are occasionally seeing is that job-pods created from subsequent cron intervals pile up over time, sometimes leading to hundreds or thousands of pods that stay pending.

We should implement a check that triggers a warning for Allow cronjobs and suggests to use one of the safer alternative modes Forbid or Replace.

[jasimmons]

Hey @timoreimann, I'm going to pick this up! 😄

[varshavaradarajan]

fixed by #97