add gerritexec example in README
add gerritexec example in README

In the section about integration tests, document the actual gerritexec
oneliner used to trigger integration tests from review.openstack.org

Change-Id: I1b980bf40f66877cbb2bc65b40a82e2a7d2d3204
Signed-off-by: Loic Dachary <b6eae37fc74638590e3f9030895d42446505d41b@dachary.org>

norcams/puppet-ceph,IPnett/puppet-ceph,MI-OSiRIS/puppet-ceph,ceph/puppet-ceph,gwdg/puppet-ceph,cloudbuy/puppet-ceph,fuel-infra/puppet-ceph,tdb/puppet-ceph,kevinlowrie/puppet-ceph,cloudbuy/puppet-ceph,aldavud/puppet-ceph,openstack/puppet-ceph,NeCTAR-RC/puppet-ceph,Emmenemoi/puppet-ceph,stackforge/puppet-ceph,openstack/puppet-ceph,rdo-puppet-modules/puppet-ceph,koder-ua/ceph,koder-ua/ceph,fuel-infra/puppet-ceph,0xf2/puppet-ceph,fultonj/puppet-ceph,muccg/puppet-ceph,norcams/puppet-ceph,MI-OSiRIS/puppet-ceph,vshn/puppet-ceph,pdrakeweb/puppet-ceph