Nomad status mixes timezones

### Nomad version

```
Nomad v0.8.7
```

### Operating system and Environment details

Linux server, Linux/OSX client.

### Issue

When issuing a `nomad status` nomad responds with datetimes in multiple timezones.

### Reproduction steps

```
$ nomad status
ID Type Priority Status Submit Date
clusterapp service 50 running 2019-03-19T00:04:22Z
diceapp service 50 running 2019-04-01T19:59:14+01:00
egress system 50 running 2019-04-01T15:18:26+01:00
ingress system 50 running 2019-03-22T00:25:49Z
whoami service 50 running 2019-03-18T16:43:23Z
```

Start by reproducing the mixed timestamp output with `nomad status` using the versions and Linux/OSX environments described in the issue. Trace the command's datetime formatting; done means the status table presents datetimes consistently in one timezone, with a regression test covering the reported output.

Written by the indexing model from the issue text.