Take 2 Global Settings and User Settings

[nhojpatrick]

Another approach to #244

Adding gitflow.maven.settings.global and gitflow.maven.settings.user as plugin properties.

If -gs is already in the argLine it is skipped, same handling for -s.

[nhojpatrick]

@aleksandr-m any thoughts about this PR, like the maven wrapper detection PR. It defines the variables the parent process automatically detected for -gs or -s, but if they are specified they are skipped. And if they are defined as a property those are used instead.

[nhojpatrick]

@aleksandr-m want to add you as a reviewer and request a review but don't have permissions to do that.

[nhojpatrick]

@aleksandr-m can this also be review and hopefully merged please. This and the other would result vm config and extra pom config.

[stefanseifert]

+1, i hit the same problem when developing the ITs for #323

[stefanseifert]

it would be good to also add an integration test with custom maven settings.
i can help with this - or get inspiration from my PR #323 where i already had a use case to pass custom maven settings within an IT