Use nginx's official repo and improve TLS configuration

These packagea are newer than the 15.04 we had. We can enable HTTP2

site-cookbooks/kosmos-nginx/recipes/default.rb

@@ -22,9 +22,26 @@ node.override['nginx']['log_formats']['json']  = <<-EOF
 '"ua":"$http_user_agent"}'
 EOF
 
-
+node.override['nginx']['repo_source'] = 'nginx' # Install from official repo
+node.override['nginx']['upstream_repository'] = "http://nginx.org/packages/mainline/#{node['platform']}"
 include_recipe 'nginx'
 
+# Generate Strong Diffie-Hellman Group (increases security)
+# https://weakdh.org/sysadmin.html
+openssl_dhparam "/etc/ssl/private/dhparams.pem" do
+  key_length 2048
+  mode 0600
+  owner 'www-data'
+end
+
+cookbook_file "#{node['nginx']['dir']}/conf.d/tls_config.conf" do
+  source 'nginx_tls_config.conf'
+  owner  'root'
+  group  'root'
+  mode   '0644'
+  notifies :restart, 'service[nginx]'
+end
+
 unless node.chef_environment == "development"
   include_recipe 'kosmos-base::firewall'