Added extra-index-url to pip optional settings

This patch ensures that the 'extra-index-url' field is available
in the global pip.conf file, so that project-config's
configure_mirrors.sh script can use sed to replace it with the
region appropriate wheel mirror.

Change-Id: Ic9b069313c7a4e105cd086bdc4516c6c5826d53e

modules/openstack_project/manifests/template.pp

@@ -240,6 +240,9 @@ class openstack_project::template (
   class { '::pip':
     index_url       => $pypi_index_url,
     trusted_hosts   => $pypi_trusted_hosts,
+    optional_settings => {
+      'extra-index-url' => '',
+    },
     manage_pip_conf => true,
   }