Add support for custom matrix subdomain

roles/custom/matrix-base/defaults/main.yml

@@ -57,7 +57,7 @@ matrix_homeserver_generic_secret_key: ''
 
 # This is where your data lives and what we set up.
 # This and the Element FQN (see below) are expected to be on the same server.
-matrix_server_fqn_matrix: "matrix.{{ matrix_domain }}"
+matrix_server_fqn_matrix: "{{ matrix_subdomain }}.{{ matrix_domain }}"
 
 # This is where you access federation API.
 matrix_server_fqn_matrix_federation: '{{ matrix_server_fqn_matrix }}'