Task Interpolation behavior change in 0.9.0

### Nomad version
```
Nomad v0.9.0 (18dd59056ee1d7b2df51256fe900a98460d3d6b9)
```
Also affects v0.9.1 and v0.9.2

### Operating system and Environment details
```
Linux linux 4.4.0-116-generic #140-Ubuntu SMP Mon Feb 12 21:23:04 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
```
Affects macOS installation too.

### Issue

Nomad 0.9.0 started interpolating variables appearing anywhere in task driver config, and failing if nomad is unable to interpolate. Nomad 0.8 jobs would interpolate if we can, but leave interpolation text unprocessed if it couldn't. This means that Nomad 0.8 jobs that used shell interpolation with `"${ENV_VAR}"` syntax stop working.

Operators can workaround behavior by referencing env-var differently, e.g. without braces (i.e. `$ENV_VAR`) or by double quoting env-var reference (e.g. `$${ENV_VAR}`).

### Reproduction steps
On 0.8.7, run the following:
```
nomad job run
nomad job status
nomad job logs
```
Then try running it against a 0.9.X release

### Job file (if appropriate)

```
job "example-envvar" {
datacenters = ["dc1"]
type = "batch"

group "cache" {
task "redis" {
driver = "docker"

config {
image = "redis:3.2"
command = "/bin/sh"
args = ["-c", < Monitoring evaluation "8628a5a5"
Evaluation triggered by job "example-envvar"
Allocation "07315953" created: node "e75e799a", group "cache"
Evaluation status changed: "pending" -> "complete"
==> Evaluation "8628a5a5" finished with status "complete"
vagrant@linux:/tmp$ nomad alloc status 07315953
ID = 07315953
Eval ID = 8628a5a5
Name = example-envvar.cache[0]
Node ID = e75e799a
Node Name =
Job ID = example-envvar
Job Version = 0
Client Status = complete
Client Description =
Desired Status = run
Desired Description =
Created = 9s ago
Modified = 3s ago

Task "redis" is "dead"
Task Resources
CPU Memory Disk Addresses
500 MHz 256 MiB 300 MiB

Task Events:
Started At = 2019-06-11T12:27:07Z
Finished At = 2019-06-11T12:27:11Z
Total Restarts = 0
Last Restart = N/A

Recent Events:
Time Type Description
2019-06-11T12:27:11Z Terminated Exit Code: 0
2019-06-11T12:27:07Z Started Task started by client
2019-06-11T12:27:06Z Task Setup Building Task Directory
2019-06-11T12:27:06Z Received Task received by client
vagrant@linux:/tmp$ nomad alloc logs 07315953
hello from envvar
```

On 0.9.2:

```
vagrant@linux:/tmp$ nomad job run /tmp/example-envvar.nomad
==> Monitoring evaluation "b91b7c61"
Evaluation triggered by job "example-envvar"
Allocation "f688d217" created: node "96861c74", group "cache"
Allocation "f688d217" status changed: "pending" -> "failed" (Failed tasks)
Evaluation status changed: "pending" -> "complete"
==> Evaluation "b91b7c61" finished with status "complete"
vagrant@linux:/tmp$ nomad alloc status f688d217
ID = f688d217
Eval ID = b91b7c61
Name = example-envvar.cache[0]
Node ID = 96861c74
Node Name = linux
Job ID = example-envvar
Job Version = 0
Client Status = failed
Client Description = Failed tasks
Desired Status = run
Desired Description =
Created = 6s ago
Modified = 2s ago
Replacement Alloc ID = 7b5bb5f8

Task "redis" is "dead"
Task Resources
CPU Memory Disk Addresses
500 MHz 256 MiB 300 MiB

Task Events:
Started At = N/A
Finished At = 2019-06-11T12:27:57Z
Total Restarts = 0
Last Restart = N/A

Recent Events:
Time Type Description
2019-06-11T12:27:57Z Killing Sent interrupt
2019-06-11T12:27:57Z Not Restarting Error was unrecoverable
2019-06-11T12:27:57Z Driver Failure failed to decode driver config: [pos 37]: readContainerLen: Unrecognized descriptor byte: hex: d4, decimal: 212
2019-06-11T12:27:57Z Task Setup Building Task Directory
2019-06-11T12:27:57Z Received Task received by client
vagrant@linux:/tmp$ nomad alloc logs f688d217
Error reading file: Unexpected response code: 404 (task "redis" not started yet. No logs available)
```