Use root user for editing pip.conf

In Icb590e805d0d29130bff99335590d1d0f646e4e4 we added a task to ensure
we don't use our own mirrors for wheel builing.  This updates pip.conf
but isn't execurted as root.  As pip.conf (and /etc) are owned by root
we need to switch to that user for this task.

Without this we see the following failures:
    http://logs.openstack.org/96/594496/2/check/build-wheel-mirror-centos-7/76fba4f/job-output.txt.gz#_2018-10-11_02_07_18_214033

Change-Id: Iee83a463aa9f3d4d45727e3f6c641ce8436b0b83

roles/build-wheels/tasks/main.yaml

@@ -7,6 +7,7 @@
   become: yes
 
 - name: Prevent using existing wheel mirror
+  become: yes
   lineinfile:
     path: /etc/pip.conf
     state: absent