Wire matrix_container_images_self_build to self_build variables via group_vars/matrix_servers

This keeps the roles cleaner and more independent of matrix-base,
which may be important for people building their own playbook
out of the individual roles and not using the matrix-base role.

roles/matrix-mxisd/defaults/main.yml

@@ -2,7 +2,8 @@
 # See: https://github.com/kamax-matrix/mxisd
 
 matrix_mxisd_enabled: true
-matrix_mxisd_self_build: "{{ matrix_container_images_self_build }}"
+
+matrix_mxisd_self_build: false
 
 matrix_mxisd_docker_image: "kamax/mxisd:1.4.6"
 matrix_mxisd_docker_image_force_pull: "{{ matrix_mxisd_docker_image.endswith(':latest') }}"