KVM/builder naming for CentOS 7 and OpenSUSE is different from other versions, minor upgrade test fails in buildbot

Old CentOS VM images/builders are called kvm-rpm-centosX-amd64; but for CentOS 7 it's kvm-rpm-centos7_0-x86_64 (x86_64 instead of amd64, and 7_0 instead of 7).
At least minor upgrade test apparently parses the name to create a baseurl to the yum repo, and ends up with a wrong one (http://yum.mariadb.org/10.0/centos7_0-x86_64).

Same for kvm-zyp-opensuse13_1-x86 and kvm-zyp-opensuse13_1-x86_64 – they also don't correspond to the repo names.