Move matrix_container_global_registry_prefix to matrix-base

Various roles depend on this. It makes sense to make it part
of the `matrix-base` role.

group_vars/matrix_servers

@@ -9,8 +9,6 @@
 # You can also override ANY variable (seen here or in any given role),
 # by re-defining it in your own configuration file (`inventory/host_vars/matrix.<your-domain>`).
 
-matrix_container_global_registry_prefix: "docker.io/"
-
 ######################################################################
 #
 # matrix-base

roles/matrix-base/defaults/main.yml

@@ -65,6 +65,8 @@ matrix_architecture: amd64
 # We just remap from our `matrix_architecture` values to what Debian and possibly other distros call things.
 matrix_debian_arch: "{{ 'armhf' if matrix_architecture == 'arm32' else matrix_architecture }}"
 
+matrix_container_global_registry_prefix: "docker.io/"
+
 matrix_user_username: "matrix"
 matrix_user_groupname: "matrix"