roles/nginx: Add missing nginx tag

The creation of the fastcgi cache dir is part of the nginx role and
should be labled as such. In situations where you only run nginx
tasks with `-t nginx` nginx will fail to start due to the missing
cache dir.

roles/nginx/tasks/main.yml

@@ -27,6 +27,7 @@
 
 - name: Create fastcgi cache dir
   file: path=/var/cache/nginx/cached/fastcgi state=directory owner=nginx group=nginx mode=0755
+  tags: nginx
 
 - include: vhosts.yml
   when: nginx_vhosts is defined