bootstrap: explain the system python-libvirt pkg
bootstrap: explain the system python-libvirt pkg

In #sepia today, Zack explained the reasoning for the python-libvirt
package on the Debian family of distros.

ktdreyer/teuthology,caibo2014/teuthology,SUSE/teuthology,yghannam/teuthology,zhouyuan/teuthology,ceph/teuthology,yghannam/teuthology,t-miyamae/teuthology,ceph/teuthology,dreamhost/teuthology,dmick/teuthology,ivotron/teuthology,michaelsevilla/teuthology,robbat2/teuthology,robbat2/teuthology,ivotron/teuthology,SUSE/teuthology,michaelsevilla/teuthology,dmick/teuthology,dreamhost/teuthology,t-miyamae/teuthology,caibo2014/teuthology,ktdreyer/teuthology,zhouyuan/teuthology,dmick/teuthology,SUSE/teuthology