Defining consul-template based on hostname environment variable

### Consul Template version

consul-template v0.16.0

### Configuration

```hcl
{{ $host := env "HOSTNAME" }}
{{ $regex := "^b" }}
{{ if $host | regexMatch "^a" }}{{ $regex := "^a" }}{{ end }}
{{ range service "service-name" }}
{{- if .Node | regexMatch $regex -}}server {{.Node}} {{.Address}}:{{.Port}}
{{ end }}{{ end }}
```

### Expected behavior
Since hostname starts with "a" - I expect to get nodes which match regex "^a".

### Actual behavior
Consul-template renders nodes which match regex "^b".

### Steps to reproduce

1. Write block with the template lines above in consul template file.
2. Run consul-template.

Start by reproducing the provided template configuration with consul-template v0.16.0 and inspect how variables assigned inside the if block are scoped. Add a regression test for the HOSTNAME values beginning with "a" and "b"; done means the rendered service nodes use the corresponding regex in each case.

Written by the indexing model from the issue text.