roles/nginx: Use template for nginx repo

A template is better than ansible's `apt_repository` module because
we can idempotently control the contents of the file based on vari-
ables.

Signed-off-by: Alan Orth <alan.orth@gmail.com>

roles/nginx/templates/nginx_org_packages_ubuntu.list.j2

@@ -0,0 +1,5 @@
+{% if nginx_version == "stable" %}
+deb http://nginx.org/packages/ubuntu/ {{ ansible_distribution_release }} nginx
+{% elif nginx_version == "mainline" %}
+deb http://nginx.org/packages/mainline/ubuntu/ {{ ansible_distribution_release }} nginx
+{% endif %}