feature: support registry mirrors

[zhuangqh]

Signed-off-by: zhuangqh zhuangqhc@gmail.com

Ⅰ. Describe what this PR did

registryMirrors is a list of registry URLs that act as a mirror for the default registry.

Ⅱ. Does this pull request fix one issue?

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

we need a new test framework to inject the deamon config.

Ⅳ. Describe how to verify it

# start pouchd with --registry-mirrors parameters
$ pouchd --registry-mirrors fake --registry-mirrors gcr.io

# pull a image without full reference
[root@pouch ~]# pouch pull cri-tools/hostnet-nginx:latest
gcr.io/cri-tools/hostnet-nginx:latest:                                            resolved       |++++++++++++++++++++++++++++++++++++++|
manifest-sha256:d629efc3a5de1fe63bac3d877ed642968add1e795260b71a40802a3e9e3c5371: done           |++++++++++++++++++++++++++++++++++++++|
layer-sha256:c59fd35c9fab19f6db27c1aeb7f4d9fb8862c8fd9400104af66bc3e589ed0ee9:    done           |++++++++++++++++++++++++++++++++++++++|
config-sha256:1e4531df184ea4708d9274d9dd8accf7352906c45cff67bdd276b68c87bfdea5:   done           |++++++++++++++++++++++++++++++++++++++|
layer-sha256:2a72cbf407d67c7a7a76dd48e432091678e297140dce050ad5eccad918a9f8d6:    done           |++++++++++++++++++++++++++++++++++++++|
layer-sha256:bce480ca577d3e311502bf1cc6192c72e2ec1d7d705ee905e9f1acf8f3c3ab10:    done           |++++++++++++++++++++++++++++++++++++++|
layer-sha256:fd4bd92a13906102906bdfb79dc15c6736f240338576b69b94f6a0078948d1d9:    done           |++++++++++++++++++++++++++++++++++++++|
elapsed: 97.2s

Ⅴ. Special notes for reviews

[codecov]

Codecov Report

Merging #2874 into master will decrease coverage by 1.13%.
The diff coverage is 86%.

@@            Coverage Diff             @@
##           master    #2874      +/-   ##
==========================================
- Coverage   69.09%   67.96%   -1.14%     
==========================================
  Files         285      285              
  Lines       17844    17887      +43     
==========================================
- Hits        12330    12157     -173     
- Misses       4114     4302     +188     
- Partials     1400     1428      +28

Flag	Coverage Δ
#criv1alpha2_test	39.07% <68%> (+0.06%)	⬆️
#integration_test_0	36.64% <76%> (+0.06%)	⬆️
#integration_test_1	35.82% <64%> (+0.06%)	⬆️
#integration_test_2	36.58% <72%> (+0.11%)	⬆️
#integration_test_3	35.54% <86%> (+0.08%)	⬆️
#node_e2e_test	?
#unittest	28.45% <0%> (-0.09%)	⬇️

Impacted Files	Coverage Δ
daemon/config/config.go	69.51% <ø> (ø)	⬆️
daemon/mgr/system.go	75.37% <100%> (+0.56%)	⬆️
ctrd/image.go	66.18% <75%> (-1.97%)	⬇️
daemon/mgr/image.go	61.2% <86.84%> (+1.75%)	⬆️
cri/stream/remotecommand/websocket.go	0% <0%> (-81.54%)	⬇️
cri/stream/remotecommand/exec.go	36.66% <0%> (-43.34%)	⬇️
cri/stream/remotecommand/errors.go	14.28% <0%> (-14.29%)	⬇️
cri/v1alpha2/cri.go	60.6% <0%> (-8.46%)	⬇️
cri/ocicni/netns.go	52.7% <0%> (-5.41%)	⬇️
daemon/logger/jsonfile/encode.go	78.26% <0%> (-3.27%)	⬇️
... and 12 more

[zhuangqh]

link to #2875

[fuweid]

LGTM