Derive secrets from matrix_homeserver_generic_secret_key, not matrix_synapse_macaroon_secret_key

We're trying to move away from implementation-specific variables,
hoping for a clean (implementation-neutral) examples/vars.yml file.

roles/matrix-base/defaults/main.yml

@@ -16,6 +16,9 @@ matrix_domain: ~
 # The homeserver implementation of an existing server cannot be changed without data loss.
 matrix_homeserver_implementation: synapse
 
+# This contains a secret, which is used for generating various other secrets later on.
+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 }}"