bugfix: extend reference separator

[fuweid]

Signed-off-by: Wei Fu [email protected]

Ⅰ. Describe what this PR did

In order to compatible with moby, need to extend oci separator with ___.

for example, localhost:5000/testing:build__20180819 should be valid. However, the ___ is invalid.

Ⅱ. Does this pull request fix one issue?

fixes #2113

Ⅲ. Why don't you add test cases (unit test/integration test)? (你真的觉得不需要加测试吗？)

Added test

Ⅳ. Describe how to verify it

Check the test case

Ⅴ. Special notes for reviews

[codecov-io]

Codecov Report

Merging #2117 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2117      +/-   ##
==========================================
+ Coverage   65.17%   65.19%   +0.01%     
==========================================
  Files         208      208              
  Lines       16411    16411              
==========================================
+ Hits        10696    10699       +3     
+ Misses       4388     4386       -2     
+ Partials     1327     1326       -1

Flag	Coverage Δ
#criv1alpha1test	33.29% <ø> (-0.02%)	⬇️
#criv1alpha2test	33.92% <ø> (-0.06%)	⬇️
#integrationtest	39.85% <ø> (+0.01%)	⬆️
#unittest	23.9% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pkg/reference/regexp.go	100% <ø> (ø)	⬆️
ctrd/image.go	77.19% <0%> (-1.76%)	⬇️
cri/v1alpha1/cri.go	63.79% <0%> (-0.35%)	⬇️
daemon/mgr/container.go	56.5% <0%> (+0.2%)	⬆️
cri/v1alpha2/cri.go	64.97% <0%> (+0.33%)	⬆️
ctrd/watch.go	80.3% <0%> (+4.54%)	⬆️
apis/server/utils.go	66.66% <0%> (+4.76%)	⬆️

[chuanchang]

@fuweid there are 2 questions in issue #2113 , this PR has only solve double underline issue in image url. the problem still exists for the following cmdline:

$ pouch pull registry.hub.docker.com/library/foo_

If the PR doesn't plan to fix above issue, and then it also LGTM.