OZG-6177 set ETR_NAMESPACE from env

...a/de/ozgcloud/operator/elstertransfer/user/Constants.java

...lstertransfer/user/OzgCloudElsterTransferUserService.java

src/main/resources/application.yml

...fer/user/OzgCloudElsterTransferUserRemoteServiceTest.java

...rtransfer/user/OzgCloudElsterTransferUserServiceTest.java