use credential helper if defined

[denisa]

Candidate for issue #806.
Opening already to get some feedback, especially any idea on how to test this…

I have only performed happy-path tests manually until now.

[codecov]

Codecov Report

Merging #811 into master will decrease coverage by 0.01%.
The diff coverage is 28.37%.

@@             Coverage Diff              @@
##             master     #811      +/-   ##
============================================
- Coverage     47.52%   47.51%   -0.02%     
- Complexity     1069     1080      +11     
============================================
  Files           134      135       +1     
  Lines          6923     6988      +65     
  Branches        910      920      +10     
============================================
+ Hits           3290     3320      +30     
- Misses         3351     3384      +33     
- Partials        282      284       +2

Impacted Files	Coverage Δ	Complexity Δ
...ric8/maven/docker/access/util/ExternalCommand.java	10.71% <0%> (+10.71%)	3 <0> (+3)	⬆️
...ric8/maven/docker/util/CredentialHelperClient.java	25% <25%> (ø)	3 <3> (?)
...o/fabric8/maven/docker/util/AuthConfigFactory.java	78.89% <47.61%> (-2.19%)	64 <0> (+3)
...ic8/maven/docker/config/RunImageConfiguration.java	86.13% <0%> (ø)	36% <0%> (ø)	⬇️
...ain/java/io/fabric8/maven/docker/util/EnvUtil.java	62.96% <0%> (ø)	39% <0%> (ø)	⬇️
...fabric8/maven/docker/AbstractBuildSupportMojo.java	0% <0%> (ø)	0% <0%> (ø)	⬇️
...er/assembly/DockerAssemblyConfigurationSource.java	17.24% <0%> (+1.14%)	9% <0%> (+1%)	⬆️
...a/io/fabric8/maven/docker/util/MojoParameters.java	75% <0%> (+2.77%)	5% <0%> (+1%)	⬆️

[rhuss]

Thanks ! I hope I can make a review soon, but can't make any promise (hard week ahead)