Switch to fully qualified RHCOS image URL

This aligns with the format expected from openshift-install, and
means we can remove some backwards compatibility from the downloader
container ref openshift-metal3/rhcos-downloader#6

common.sh

@@ -47,7 +47,7 @@ export NUM_MASTERS=${NUM_MASTERS:-"3"}
 export NUM_WORKERS=${NUM_WORKERS:-"1"}
 export VM_EXTRADISKS=${VM_EXTRADISKS:-"false"}
 
-export RHCOS_INSTALLER_IMAGE_URL="https://releases-art-rhcos.svc.ci.openshift.org/art/storage/releases/rhcos-4.2/420.8.20190708.2/"
+export RHCOS_INSTALLER_IMAGE_URL="https://releases-art-rhcos.svc.ci.openshift.org/art/storage/releases/rhcos-4.2/420.8.20190708.2/rhcos-420.8.20190708.2-openstack.qcow2"
 export RHCOS_IMAGE_URL=${RHCOS_IMAGE_URL:-${RHCOS_INSTALLER_IMAGE_URL}}
 export RHCOS_IMAGE_FILENAME_LATEST="rhcos-ootpa-latest.qcow2"