Support global registry configuration

[ahmed-mez]

What does this PR do?

Support a global registry config.

Motivation

Configure the container registry without depending on the image name and tag.

Additional Notes

For backward compatibility, the tag and registry values are ignored in case Image.Name corresponds to a full image name.

Describe your test plan

Example CR:

apiVersion: datadoghq.com/v1alpha1
kind: DatadogAgent
metadata:
  name: datadog
spec:
  registry: public.ecr.aws/datadog
...
  agent:
    image:
      name: "agent"
      tag: "latest"
  clusterAgent:
    replicas: 1
    image:
      name: "cluster-agent"
      tag: "latest"
    config:
      clusterChecksEnabled: true
  clusterChecksRunner:
    replicas: 1
    image:
      name: "agent"
      tag: "latest"

[clamoriniere]

Few ideas to provide more options to the user.

[codecov-commenter]

Codecov Report

Merging #309 (6496b55) into main (023cb3b) will increase coverage by 0.18%.
The diff coverage is 76.47%.

@@            Coverage Diff             @@
##             main     #309      +/-   ##
==========================================
+ Coverage   58.18%   58.36%   +0.18%     
==========================================
  Files          59       59              
  Lines        6294     6288       -6     
==========================================
+ Hits         3662     3670       +8     
+ Misses       2362     2348      -14     
  Partials      270      270              

Flag	Coverage Δ
unittests	58.36% <76.47%> (+0.18%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
api/v1alpha1/datadogagent_default.go	14.58% <0.00%> (+0.51%)	⬆️
api/v1alpha1/datadogagent_types.go	100.00% <ø> (ø)
controllers/datadogagent/clusteragent.go	69.35% <100.00%> (ø)
controllers/datadogagent/clusterchecksrunner.go	75.79% <100.00%> (+0.06%)	⬆️
controllers/datadogagent/utils.go	82.02% <100.00%> (+0.09%)	⬆️

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[vboulineau]

It will conflict a bit with #303 (see usage of GetTagFromImageName)

[ahmed-mez]

@vboulineau do you want to merge #303 first?