refactor: make time format compatible with Moby

[zeppp]

Signed-off-by: zeppp [email protected]

1.Describe what this PR did
The time fields such as Created and StartAt in Moby are both formatted by RFC3339Nano which is different from Pouch.
So I change the time format in Pouch to keep consistence with Moby.

2.Does this pull request fix one issue?

3.Describe how you did it

4.Describe how to verify it

>> # pouch ps
Name   ID       Status          Created          Image                                            Runtime
2      961798   Up 12 minutes   13 minutes ago   registry.hub.docker.com/library/busybox:latest   runc
>> # pouch inspect 2
{
  "Args": null,
  "Config": {
    "Cmd": [
      "sh"
    ],
    "Entrypoint": null,
    "Env": [
      "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
    ],
    "Image": "registry.hub.docker.com/library/busybox:latest",
    "OnBuild": null,
    "Shell": null
  },
➡️  "Created": "2018-01-11T06:04:07.189360689Z",
  "HostConfig": {
    "Runtime": "runc",
    "BlkioDeviceReadBps": null,
    "BlkioDeviceReadIOps": null,
    "BlkioDeviceWriteBps": null,
    "BlkioDeviceWriteIOps": null,
    "BlkioWeightDevice": null,
    "DeviceCgroupRules": null,
    "Devices": [],
    "MemorySwappiness": -1,
    "Ulimits": null
  },
  "Id": "961798b0b8fa9b161863ec08eaf020293766a8758805e63c8e3f443c74bb5ee6",
  "Image": "registry.hub.docker.com/library/busybox:latest",
  "Mounts": null,
  "Name": "2",
  "State": {
    "Pid": 5931,
➡️    "StartedAt": "2018-01-11T06:04:26.971241293Z",
    "Status": "running"
  }
}

5.Special notes for reviews
Please remove old containers before applying this PR.
@allencloud

[allencloud]

Thanks a lot for your work. 🌹 🧀

Please remove old containers before applying this PR.

I will and this is reasonable.

[codecov-io]

Codecov Report

Merging #558 into master will decrease coverage by 0.09%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master    #558     +/-   ##
========================================
- Coverage   14.49%   14.4%   -0.1%     
========================================
  Files          64      64             
  Lines        3173    3172      -1     
========================================
- Hits          460     457      -3     
- Misses       2666    2668      +2     
  Partials       47      47

Impacted Files	Coverage Δ
pkg/utils/utils.go	92.1% <ø> (-5.34%)	⬇️

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[allencloud]

LGTM