Support for multiple scoped resources using the same template

I needed to be able to generate various configuration files form the same template and config structure changing the scope of the resource, so I added a resource prefix.

Let me know if this is useful.

Copying the documentation:

# Templates - Interation Example

Using confd to manage nginx proxy config of several apps in subdomains

## Add two apps with upstream servers to etcd

```Bash
# Config for myapp
curl http://127.0.0.1:4001/v2/keys/myapp/upstream -XPUT -d dir=true
curl http://127.0.0.1:4001/v2/keys/myapp/subdomain -XPUT -d value="myapp"
curl http://127.0.0.1:4001/v2/keys/myapp/upstream/app2 -XPUT -d value="10.0.1.101:80"
curl http://127.0.0.1:4001/v2/keys/myapp/upstream/app1 -XPUT -d value="10.0.1.100:80"
# Config for yourapp
curl http://127.0.0.1:4001/v2/keys/yourapp/upstream -XPUT -d dir=true
curl http://127.0.0.1:4001/v2/keys/yourapp/subdomain -XPUT -d value="yourapp"
curl http://127.0.0.1:4001/v2/keys/yourapp/upstream/app2 -XPUT -d value="10.0.1.103:80"
curl http://127.0.0.1:4001/v2/keys/yourapp/upstream/app1 -XPUT -d value="10.0.1.102:80"
```

## Create template resources

`/etc/confd/conf.d/myapp-nginx.toml`

```TOML
[template]
prefix = "myapp"
keys = [
  "subdomain",
  "upstream",
]
owner = "nginx"
mode = "0644"
src = "nginx.tmpl"
dest = "/tmp/myapp.conf"
check_cmd  = "/usr/sbin/nginx -t -c {{ .src }}"
reload_cmd = "/usr/sbin/service nginx reload"
```

`/etc/confd/conf.d/yourapp-nginx.toml`

```TOML
[template]
prefix = "yourapp"
keys = [
  "subdomain",
  "upstream",
]
owner = "nginx"
mode = "0644"
src = "nginx.tmpl"
dest = "/tmp/yourapp.conf"
check_cmd  = "/usr/sbin/nginx -t -c {{ .src }}"
reload_cmd = "/usr/sbin/service nginx reload"
```

## Create a source template

`/etc/confd/templates/nginx.tmpl`

```
upstream {{.subdomain}} {
{{range $server := .upstream}}
    server {{$server.Value}};
{{end}}
}

server {
    server_name  {{.subdomain}}.example.com;

    location / {
        proxy_pass        http://{{.subdomain}};
        proxy_redirect    off;
        proxy_set_header  Host             $host;
        proxy_set_header  X-Real-IP        $remote_addr;
        proxy_set_header  X-Forwarded-For  $proxy_add_x_forwarded_for;
   }
}
```