Make federation domain customizable

roles/matrix-base/defaults/main.yml

@@ -11,6 +11,9 @@ matrix_domain: ~
 # This and the Element FQN (see below) are expected to be on the same server.
 matrix_server_fqn_matrix: "matrix.{{ matrix_domain }}"
 
+# This is where you access federation API.
+matrix_server_fqn_matrix_federation: '{{ matrix_server_fqn_matrix }}'
+
 # This is where you access the Element web UI from (if enabled via matrix_client_element_enabled; enabled by default).
 # This and the Matrix FQN (see above) are expected to be on the same server.
 matrix_server_fqn_element: "element.{{ matrix_domain }}"