elk7: init at 7.0.0

This adds the following new packages:

+ elasticsearch7
+ elasticsearch7-oss
+ logstash7
+ logstash7-oss
+ kibana7
+ kibana7-oss
+ filebeat7
+ heartbeat7
+ metricbeat7
+ packetbeat7
+ journalbeat7

The default major version of the ELK stack stays at 6. We should
probably set it to 7 in a next commit.