Save about 14MB of image size

[aleksey-mashanov]

Running rpm --rebuilddb before generating tar archive will save about 14MB of image size.

$ docker run -it --rm centos:7
[root@97deac1d468c /]# du -hs /var/lib/rpm
22M	/var/lib/rpm
[root@97deac1d468c /]# rpm --rebuilddb
[root@97deac1d468c /]# du -hs /var/lib/rpm
8.4M	/var/lib/rpm

The same issue in the images repo: CentOS/sig-cloud-instance-images#155