Skip overwriting existing service definition

[podhy]

Hello,
this PR disables overwriting of existing services definition and just sets autowire option to true.

This is very usefull when you want to pass e.g. scalar parametrs from parameters.yml into controller because in SF 2.8.3 you can do:

controller_service:
    class: SomeController
    arguments:
        - ''
        - %scalar_parametr%

And in upcomming SF 2.8.4:

controller_service:
    class: SomeController
    arguments:
        1: %scalar_parametr%

So the first parametr is autowired and the second is not

[TomasVotruba]

Hi @podhy,

thanks for the improvement and adding tests. They really nicely cover the feature.