Install parallel from EPEL for build-wheel-mirror

Right now, it seems we only need parallel from EPEL. Rather then
enabling the whole repo, we can just opt to install it first, then
bindep will be happy.

Change-Id: Ifd1eb186f78c395f549d1430cfe4414ac1121f63
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

playbooks/wheel/pre.yaml

@@ -5,3 +5,14 @@
       bindep_dir: "src/{{ zuul.project.canonical_name }}"
     - legacy-install-afs-with-puppet
     - legacy-copy-project-config-scripts
+
+  tasks:
+    # NOTE(pabelanger): We should consider switching away from parallel so we
+    # don't need to enable EPEL for centos 7.
+    - name: Manually install parallel from EPEL
+      become: yes
+      yum:
+        name: parallel
+        enablerepo: epel
+        state: present
+      when: "{{ ansible_distribution }} == CentOS"