Using vault values in health checks

### Nomad version
Nomad v0.11.1 (b43457070037800fcc8442c8ff095ff4005dab33)

### Operating system and Environment details
Ubuntu 18.04

### Issue
It doesn't seem possible to use vault values such as passwords in defining service health checks - such as Authorization header values for http health checks.

From what I can tell, Vault values are only available via consul-template which is only allowed to write files or set env vars in the target container. Otoh, the service health checks don't have visibility of the environment vars set in the target container.

### Reproduction steps
I've tried various options and nothing has worked. Note that normal interpolation - such as `${NOMAD_GROUP_NAME}` *does* work and that's not what I'm reporting here.

### Job file (if appropriate)
This is how I feel it could work (but it doesn't). This example tries to setup the basic auth header for an http based health check. First we create the base64 value:
```
template {
change_mode = "noop"
data = <