Rename a bunch of playbook variables for better consistency

roles/matrix-server/tasks/import_sqlite_db.yml

@@ -52,7 +52,7 @@
 - name: Importing SQLite database into Postgres
   docker_container:
     name: matrix-synapse-migrate
-    image: "{{ docker_matrix_image }}"
+    image: "{{ matrix_docker_image_synapse }}"
     detach: no
     cleanup: yes
     entrypoint: /usr/bin/python
@@ -69,4 +69,4 @@
 - name: Ensure scratchpad directory is deleted
   file:
     path: "{{ matrix_scratchpad_dir }}"
-    state: absent
+    state: absent