SUSE-SU-2019:1973-1 Security update for rmt-server

This update for rmt-server to version 2.3.1 fixes the following issues: - Fix mirroring logic when errors are encountered bsc1140492 - Refactor RMT::Mirror to download metadata/licenses in parallel - Check repo metadata GPG signatures during mirroring bsc1132690 - Add rmt-server-config subpackage...