ci-workers: Variable overrides and process tweaks for CentOS 7

Part of #2510

contrib/ci-workers/unix.yml

@@ -84,6 +84,10 @@
         state: present
       with_items: "{{ package_deps }}"
 
+    - name: Install pip [CentOS]
+      include: tasks/install-pip.yml
+      when: ansible_distribution == 'CentOS'
+
     - name: Install required Python modules
       pip:
         name: "{{ item }}"