RFC: templates feature

After tinet up. this feature creates files on docker containers with `html/template`.

```yaml
---
nodes:
- name: hoge
  image: hoge
  vars:
    bgp_on: "yes"
    out: "test text"
  templates:
  - dst: /etc/frr/daemons
    src: daemons.j2
  - dst: /etc/out.txt
    content: |
      test "{{ .out }}"
...
```