Improve readability using stream chaining

[LOG-INFO]

Thanks for resolving https://github.com/spring-cloud/spring-cloud-deployer-kubernetes/pull/482 ! :D

I suggest a little refactoring for readability.

• name of variable commandArgOptions -> keyValueTypeCommandArgs
• use stream chaining
  • filter only key-value type args
  • extract keys
  • trim and remove --

I checked the tests testCommandLineArgsOverridesExistingProperties(), testCommandLineArgsExcludesMalformedProperties(), testCommandLineArgsNoAssignment() in DefaultContainerFactoryTests were passed.

cc. @onobc @cppwfs