Introduce property for Jenkins user (`user.name`) and Artifactory server details

`Jenkinsfile` contains very little environmental details. However, we still have environment-specific data in the config:

* `-Duser.name=spring-builds+jenkins`
* `-Dartifactory.server=https://repo.spring.io`
* `-Dartifactory.staging-repository=libs-snapshot-local`

We should introduce properties to `ci/pipeline.properties` and take values from there.