ci: discard old builds

Until now builds were kept forever or until manual removal. That
caused an issue with running out of space occasionally.

With this change all builds older than 30 days are discarded.
The second limit removes oldest builds to keep the history
up to 70 builds.

Reviewed-by: Justin Stephenson <jstephen@redhat.com>
Reviewed-by: Pavel Březina <pbrezina@redhat.com>

Jenkinsfile

@@ -254,6 +254,10 @@ def notification = new Notification(
   on_demand
 )
 
+this.properties([
+    buildDiscarder(logRotator(daysToKeepStr: '30', numToKeepStr: '70')),
+])
+
 try {
   stage('Get system list') {
     node('master') {