ensure-pip: add instructions for RedHat system

This change adds instructions to help user figure out what to do when
unittest job fail since https://review.opendev.org/736070 requires
epel to be installed.

Change-Id: Ie69ba81405bc3fbc7666c7ee12ed4996c5323709

roles/ensure-pip/tasks/RedHat.yaml

@@ -7,15 +7,30 @@
     state: present
   become: yes
 
-- name: Install Python 2 pip
-  yum:
-    name:
-      - python-pip
-      - python-setuptools
-      - python-virtualenv
-      - python-wheel
-    state: present
-    enablerepo: epel
+- block:
+    - name: Check for EPEL repository
+      stat:
+        path: /etc/yum.repos.d/epel.repo
+      register: _epel_repo
+
+    - name: Fail with instructions when EPEL is not installed
+      fail:
+        msg: |
+          The role `ensure-pip` cannot continue on this nodeset.
+          Install epel-release or set this Zuul job variable:
+
+             ensure_pip_from_packages_with_python2: false
+      when: not _epel_repo.stat.exists
+
+    - name: Install Python 2 pip
+      yum:
+        name:
+          - python-pip
+          - python-setuptools
+          - python-virtualenv
+          - python-wheel
+        state: present
+        enablerepo: epel
   become: yes
   when: ensure_pip_from_packages_with_python2 or
         ansible_python.version.major == 2