[BUG] `current` user fails if the group/user exists in the image already

If running as `current` user and the group or user already exists in the image, then the attempt to create the group or user will fail and error out the build.

Probably can update [`createUserAndGroup`](https://github.com/gpuopenanalytics/remote-docker-plugin/blob/master/src/main/java/com/gpuopenanalytics/jenkins/remotedocker/config/UserConfigItem.java#L143-L160) to check if it exists first.