Need option for 0 exit code when nomad plan creates/destroys allocations

As mentioned in the [docs](https://www.nomadproject.io/docs/commands/job/plan.html), **nomad plan** will return an exit code of **1** when allocations will be created or destroyed. We might want to consider a way to let the user have **0** returned in those situations. This can help where **nomad plan** is being used through CI/CD (so the pipeline doesn't actually see **nomad plan** as a failure just because allocations are being changed).