specs-go/config: Make Linux and Solaris omitempty (again)
specs-go/config: Make Linux and Solaris omitempty (again)

I'd added some omitempties in 5c2193f3 (specs-go/config: Make Linux
and Solaris omitempty, 2016-05-06, #431), but it turns out to not have
the intended effect unless the field is also a pointer type (even
after I shifted the 'omitempty' from the platform tag to the json
tag).  Before this commit:

  $ ./ocitools generate --template <(echo '{}')
  $ jq . config.json
  {
    "ociVersion": "1.0.0-rc1-dev",
    "platform": {
      "os": "linux",
      "arch": "amd64"
    },
    "process": {
      "user": {
        "uid": 0,
        "gid": 0
      },
      "args": [],
      "cwd": "/"
    },
    "root": {
      "path": "rootfs"
    },
    "hooks": {},
    "linux": {
      "cgroupsPath": ""
    },
    "solaris": {
      "cappedCPU": {},
      "cappedMemory": {}
    }
  }

And after this commit:

  $ ./ocitools generate --template <(echo '{}')
  $ jq . config.json
  {
    "ociVersion": "1.0.0-rc1-dev",
    "platform": {
      "os": "linux",
      "arch": "amd64"
    },
    "process": {
      "user": {
        "uid": 0,
        "gid": 0
      },
      "args": [],
      "cwd": "/"
    },
    "root": {
      "path": "rootfs"
    },
    "hooks": {},
  }

The remaining useless properties are addressed by other in-flight pull
requests:

* 5ca74df (config: Make 'process.args' optional, 2016-06-04, #489)
* ad33f9c (config: Explicitly list 'hooks' as optional, 2016-05-06,
  #427)

So I've left them alone here.

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

opencontainers/runtime-spec,opencontainers/specs,wking/opencontainer-runtime-spec,jonboulle/specs,hqhq/specs,cloudfoundry-incubator/specs,wking/opencontainer-specs,laijs/specs,vbatts/specs,hqhq/runtime-spec,ujuc/specs