site-config: Remove guest OS that are not publicly available

We had Windows 7 and RHEL 6.3, that not all people have
access to. So let's replace them by Fedora 18.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
client.shared.version: Fix bug on get_git_describe

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>