Install virtualenv < 20.0.19 in CentOS7

20.0.19 has caused issue[1] when running in CentOS7,
until it's fixed in next release and is in nodepool
images, let's workaround by installing older version.

[1] pypa/virtualenv#1810
Change-Id: I065c96a3f871f477ce572a1e508d489437af7d9c

playbooks/base/pre.yaml

@@ -15,3 +15,10 @@
       when:
         - _worker_is_sudoer.rc == 0
         - ansible_distribution != "Fedora" or (ansible_distribution == "Fedora" and ansible_distribution_major_version|int > 28)
+
+    # until /~https://github.com/pypa/virtualenv/issues/1810 is fixed and is in nodepool images
+    - name: install virtualenv < 20.0.19 in CentOS 7
+      command: sudo pip install 'virtualenv<20.0.19'
+      when:
+        - _worker_is_sudoer.rc == 0
+        - ansible_distribution == "CentOS" and ansible_distribution_major_version|int == 7