Install twine in executor Ansible environments

We want this pre-installed so that release jobs are more reliable.

Change-Id: I580fc046b6f58005db3ba62f6d62aeb855cb58eb

manifests/executor.pp

@@ -42,7 +42,7 @@ class zuul::executor (
 
   exec { 'zuul_manage_ansible':
     command     => 'zuul-manage-ansible',
-    environment => ['ANSIBLE_EXTRA_PACKAGES=gear'],
+    environment => ['ANSIBLE_EXTRA_PACKAGES=gear twine'],
     path        => '/usr/local/bin:/usr/bin:/bin/',
     subscribe   => Exec['install_zuul'],
     refreshonly => true,