Support gNMI SET RPC

[tanakatakafumi]

Example

• server

$ docker run -it --rm -v /etc/oopt:/etc/oopt -p 10164:10164 oopt go run /go/src/github.com/osrg/oopt/gnmi/sample_server/run.go

• client
  Update frequency channel of Opt6 to 2.

$ go run $GOPATH/src/github.com/osrg/oopt/cmd/gnmi_set/gnmi_set.go -target_addr localhost:10164 -update /optical-modules/optical-module[name=Opt6]/optical-module-frequency/config/channel:2
== setRequest:
update: <
  path: <
    elem: <
      name: "optical-modules"
    >
    elem: <
      name: "optical-module"
      key: <
        key: "name"
        value: "Opt6"
      >
    >
    elem: <
      name: "optical-module-frequency"
    >
    elem: <
      name: "config"
    >
    elem: <
      name: "channel"
    >
  >
  val: <
    int_val: 2
  >
>

== getResponse:
response: <
  path: <
    elem: <
      name: "optical-modules"
    >
    elem: <
      name: "optical-module"
      key: <
        key: "name"
        value: "Opt6"
      >
    >
    elem: <
      name: "optical-module-frequency"
    >
    elem: <
      name: "config"
    >
    elem: <
      name: "channel"
    >
  >
  op: UPDATE
>

[ishidawataru]

@tanakatakafumi Please open a PR against master branch. You can squash my commit for the fix.

[tanakatakafumi]

I changed this PR to osrg:master and squashed some commits.

[ishidawataru]

@tanakatakafumi Sorry for the delay. Merged.