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_mxisd.yml

@@ -23,7 +23,7 @@
 
 - name: Ensure mxisd image is pulled
   docker_image:
-    name: "{{ matrix_docker_image_mxisd }}"
+    name: "{{ matrix_mxisd_docker_image }}"
   when: matrix_mxisd_enabled
 
 - name: Ensure mxisd config installed
@@ -69,6 +69,6 @@
 
 - name: Ensure mxisd Docker image doesn't exist
   docker_image:
-    name: "{{ matrix_docker_image_mxisd }}"
+    name: "{{ matrix_mxisd_docker_image }}"
     state: absent
   when: "not matrix_mxisd_enabled"