Nomad hard links prevent cron from running

### Nomad version

Output from `nomad version`
nomad 0.4.0
### Operating system and Environment details

ubuntu 14.04
### Issue

Nomad prevents cron from running after creating chroot.
### Reproduction steps
1. create a chroot (via exec)
2. run a cron
3. observe that cron does not run
### Nomad Client logs (if appropriate)

From syslog:

```
Jul 29 23:03:01 nomad-agent1 cron[1043]: (*system*) NUMBER OF HARD LINKS > 1 (/etc/crontab)
Jul 29 23:03:01 nomad-agent1 cron[1043]: (*system*ansible-pull) NUMBER OF HARD LINKS > 1 (/etc/cron.d/ansible-pull)
```

Start by reproducing the chroot created through exec on Ubuntu 14.04, then inspect the cron behavior and syslog entries for /etc/crontab and /etc/cron.d/ansible-pull. Trace how the chroot handles those files; done means cron runs without reporting that their hard-link count is greater than one.

Written by the indexing model from the issue text.