Chained/nested templates

### Use-cases
We are working on a configs/secrets convention in the Nomad+Consul+Vault stack, and we would like to have the following features:
- Hot reload for config (don't store production configs in the repo/image).
- Store secrets separate from config values (in Vault).
- Don't store each config value separately, as there are too many config values.
- Don't put full config into Vault.

### Attempted Solutions

We found that the ideal solution would be a nested template. We'd like to store the config template in Consul K/V, like this:
```
{
"foo": "bar",
"secret" : "{{with secret "secret/data/nomad/nested-template-test-secrets"}}{{.Data.data.secret}}{{end}}"
}
```
Fetch it from Consul and **use as a template again** to resolve Vaulted values, like this:

```hcl
job "nested-template-test" {
datacenters = ["dc"]
group "nested-template-test" {
task "nested-template-test" {
driver = "docker"
config {
image = "alpine"
interactive = true
}

template {
data = <<-EOF
{{key "nested-template-test-config"}}
EOF
destination = "local/from-consul.json.tmpl"
}
template {
source = "local/from-consul.json.tmpl"
destination = "secrets/from-vault.json"
}

}
}
}
```
### Proposal

Unfortunately, this does not work, looks like Nomad checks all template sources before starting to execute templates.

```
template: failed to read template: exit status 1: failed to open source file "/nested-template-test/local/from-consul.json.tmpl": open /nested-template-test/local/from-consul.json.tmpl: no such file or directory
```

Maybe someone have something else to propose, which may suite our needs...