common-tasks: Do not sync preconfigured /etc/pip.conf file

Zuul places a /etc/pip.conf file on the host as shown below

2018-05-02 11:48:59.945452 | TASK [configure-mirrors : Install /etc/pip.conf configuration]

However, we don't need that since we have our own role to configure pip
which overrides the Zuul one anyway. For the hosts which are not
configured with the pip_install role, this can actually cause problems
so we wipe the file and we let our own role configure it the way we want
to.

Change-Id: I45a900b056db1c8a173f9ed1dae1bd998915e9d4
(cherry picked from commit c2030ef974)

common-tasks/test-set-nodepool-vars.yml

@@ -50,8 +50,6 @@
       set_fact:
         lxc_image_cache_server_mirrors:
           - "http://{{ lxc_reverse_proxy }}:8080/images.linuxcontainers/"
-        lxc_container_cache_files:
-          - { src: '/etc/pip.conf', dest: '/etc/pip.conf' }
         # CentOS mirrors are set to OpenStack's CI mirrors, so fastestmirror is
         # disabled to speed up package downloads.
         openstack_hosts_enable_yum_fastestmirror: no