Service jobs should have unique allocation Names, derived from the
Job.ID. System jobs do not have unique allocation Names because the index is
intended to indicated the instance out of a desired count size. Because system
jobs do not have an explicit count but the results are based on the targeted
nodes, the index is less informative and this was intentionally omitted from the
original design.
Update docs to make it clear that NOMAD_ALLOC_INDEX is always zero for
system/sysbatch jobs
Validate that `volume.per_alloc` is incompatible with system/sysbatch jobs.
System and sysbatch jobs always have a `NOMAD_ALLOC_INDEX` of 0. So
interpolation via `per_alloc` will not work as soon as there's more than one
allocation placed. Validate against this on job submission.
