Rename playbook variables so they are consistently prefixed

Pretty much all variables live in their own `matrix_<whatever>`
prefix now and are grouped closer together in the default
variables file (`roles/matrix-server/defaults/main.yml`).

roles/matrix-server/tasks/setup/setup_nginx_proxy.yml

@@ -36,7 +36,7 @@
 #
 - name: Ensure nginx Docker image is pulled
   docker_image:
-    name: "{{ matrix_docker_image_nginx }}"
+    name: "{{ matrix_nginx_proxy_docker_image }}"
   when: matrix_nginx_proxy_enabled
 
 - name: Allow access to nginx proxy ports in firewalld