Add unit-test for ValidateCPUPeriod

[johanzhu]

Ⅰ. Issue Description

Add unit-test for ValidateCPUPeriod method which locate on apis/opts/cpu.go.
fixes #1637

Ⅱ. Describe what happened

Add unit-test for ValidateCPUPeriod method which locate on apis/opts/cpu.go

Ⅲ. Describe what you expected to happen

All test case passed.

Ⅳ. How to reproduce it (as minimally and precisely as possible)

Ⅴ. Anything else we need to know?

Baiji 262 group 2.

Ⅵ. Environment:
pouch version (use pouch version):go1.10.3 darwin/amd64
OS (e.g. from /etc/os-release): CentOS linux 7
Kernel (e.g. uname -a):3.10.0
Install tools: visualbox git go
Others:none

[pouchrobot]

We found this is your first time to contribute to Pouch, @johanzhu
👏 We really appreciate it.
Just remind that you have read the contribution guide: https://github.com/alibaba/pouch/blob/master/CONTRIBUTING.md
If you didn't, you should do that first. If done, welcome again and please enjoy hacking! 🍻

[pouchrobot]

@johanzhu Thanks for your contribution. 🍻
Please sign off in each of your commits.

[codecov-io]

Codecov Report

Merging #1690 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1690      +/-   ##
==========================================
+ Coverage   41.16%   41.19%   +0.02%     
==========================================
  Files         274      274              
  Lines       18091    18091              
==========================================
+ Hits         7447     7452       +5     
+ Misses       9729     9726       -3     
+ Partials      915      913       -2

Impacted Files	Coverage Δ
ctrd/image.go	75.27% <0%> (-2.75%)	⬇️
daemon/mgr/system.go	75% <0%> (-1.73%)	⬇️
daemon/mgr/container.go	50.23% <0%> (+0.15%)	⬆️
daemon/logger/jsonfile/utils.go	73.17% <0%> (+1.62%)	⬆️
apis/server/utils.go	66.66% <0%> (+4.76%)	⬆️
apis/opts/cpu.go	50% <0%> (+50%)	⬆️

[allencloud]

LGTM