test: add unit-test for ValidateCPUQuota

[johanzhu]

Ⅰ. Issue Description
Add unit-test for ValidateCPUQuota method which locate on apis/opts/cpu.go.
#1638
Ⅱ. Describe what happened
Add unit-test for ValidateCPUQuota 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 #1692 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1692      +/-   ##
==========================================
+ Coverage   41.16%   41.21%   +0.04%     
==========================================
  Files         274      274              
  Lines       18091    18091              
==========================================
+ Hits         7447     7456       +9     
+ Misses       9729     9723       -6     
+ Partials      915      912       -3

Impacted Files	Coverage Δ
daemon/mgr/system.go	75% <0%> (-1.73%)	⬇️
ctrd/image.go	77.47% <0%> (-0.55%)	⬇️
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