Populate /etc/yum/vars/contentdir

The file /etc/yum.repos.d/CentOS-QEMU-EV.repo is assuming that
$contentdir is being populated, but it is not until new centos base
images include it.

This change detects the package architecture and populates contentdir
with 'centos' or 'altarch' as appropriate.

There is a kolla change equivalent here Ieb26c0aa65ba3ec1a45a33f87f670e5ecb782f82
The upstream discussion for adding contentdir is here:
https://lists.centos.org/pipermail/centos-devel/2018-March/016542.html

Change-Id: I6279bd4088bd87f37c58b7da25b675c5456506d6

roles/nodepool-setup/tasks/main.yml

@@ -25,6 +25,32 @@
     dest: /etc/ci/mirror_info.sh
     src: mirror_info.sh.j2
 
+- name: Check for /etc/yum/vars/contentdir
+  stat:
+    path: /etc/yum/vars/contentdir
+  register: yum_contentdir
+
+- when: not yum_contentdir.stat.exists
+  block:
+
+  - name: Discover package architecture
+    command: rpm -q --qf "%{arch}" -f /etc/redhat-release
+    register: rpm_arch
+
+  - debug:
+      msg: Package architecture is '{{ rpm_arch.stdout }}'
+
+  - name: Set contentdir to altarch
+    set_fact:
+      yum_contentdir: altarch
+    when: rpm_arch.stdout in ['aarch64', 'ppc64le']
+
+  - name: Populate /etc/yum/vars/contentdir
+    copy:
+      dest: /etc/yum/vars/contentdir
+      content: "{{ yum_contentdir|default('centos') }}"
+    become: true
+
 - name: Install repos (will be removed by toci_gatetest)
   package:
     name: "{{ item }}"