fix dst file conflict in template function

[bobuhiro11]

If a template with the same dst is applied to multiple nodes, the file name for dst would conflict.

$ cat <<EOF >spec.yaml
nodes:
  - name: h1
    type: docker
    image: alpine
    vars:
      name: h1
    templates:
    - dst: /tmp/baz.txt
      content: "{{ .name }}"
  - name: h2
    type: docker
    image: alpine
    vars:
      name: h2
    templates:
    - dst: /tmp/baz.txt
      content: "{{ .name }}"
EOF

$ go run . up | sudo sh -x
+ docker run -td --net none --name h1 --rm --privileged --hostname h1 -v /tmp/tinet:/tinet alpine
+ mkdir -p /var/run/netns
+ docker inspect h1 --format {{.State.Pid}}
+ PID=12938
+ ln -s /proc/12938/ns/net /var/run/netns/h1
+ docker run -td --net none --name h2 --rm --privileged --hostname h2 -v /tmp/tinet:/tinet alpine
+ mkdir -p /var/run/netns
+ docker inspect h2 --format {{.State.Pid}}
+ PID=13020
+ ln -s /proc/13020/ns/net /var/run/netns/h2
+ ip netns del h1
+ docker cp baz.txt h1:/tmp/baz.txt
+ rm -rf baz.txt
+ ip netns del h2
+ docker cp baz.txt h2:/tmp/baz.txt
lstat /home/bobuhiro11/tinet/baz.txt: no such file or directory
+ rm -rf baz.txt