ci: don't use CentOS Alpha anymore

It's no longer being built and is now older than the latest CentOS AH
release. This should help us no longer see messages like:

(rpm-ostree pkg-add:5662): GLib-CRITICAL **: g_variant_dict_lookup:
assertion 'is_valid_dict (dict)' failed

which happen because in coreos#1034, we started using `G_VARIANT_DICT_INIT`,
whose special magic values only make sense in glib2 >= 2.50. (The alpha
image stopped at 2.46).

Saw this while debugging coreos#1035.

.papr.yml

@@ -49,11 +49,11 @@ required: true
 cluster:
   hosts:
     - name: vmcheck1
-      distro: centos/7/atomic/alpha
+      distro: centos/7/atomic
     - name: vmcheck2
-      distro: centos/7/atomic/alpha
+      distro: centos/7/atomic
     - name: vmcheck3
-      distro: centos/7/atomic/alpha
+      distro: centos/7/atomic
   container:
     # FIXME remove this version binding when rpm-md repos are updated
     image: registry.centos.org/centos/centos:7.3.1611