Make Artifactory respect Jenkins' artifact retention policy, if set

In my Maven projects, if I configure Jenkins to retain e.g. 6 months' of build logs, but only artifacts for the last 10 builds, Artifactory retains 6 months' worth of snapshots, which is not what I would expect to happen. The change is to accept Jenkins' artifact retention policy, if set, for the purpose of setting retention on the Artifactory side.