Make matrix_homeserver_implementation influence matrix_IMPLEMENTATION_enabled, not the other way around

Doing this seems more reasonable and simpler.

roles/matrix-base/defaults/main.yml

@@ -8,11 +8,13 @@
 matrix_domain: ~
 
 # This will contain the homeserver implementation that is in use.
-# Example values: 'synapse', 'dendrite', etc.
-# You normally don't need to set this variable manually.
-# Its value is automatically set depending on the homeserver implementation that you have enabled via other variables
-# (e.g. `matrix_synapse_enabled`, `matrix_dendrite_enabled`, etc.)
-matrix_homeserver_implementation: ''
+# Valid values: synapse, dendrite
+#
+# By default, we use Synapse, because it's the only full-featured Matrix server at the moment.
+#
+# This value automatically influences other variables (`matrix_synapse_enabled`, `matrix_dendrite_enabled`, etc.).
+# The homeserver implementation of an existing server cannot be changed without data loss.
+matrix_homeserver_implementation: synapse
 
 # 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.

roles/matrix-base/tasks/sanity_check.yml

@@ -1,15 +1,9 @@
 ---
 
-- name: Fail if 0 or more than 1 homeserver implementations enabled
+- name: Fail if invalid homeserver implementation
   fail:
-    msg: >-
-      You have 0 or more than 1 homeserver implementations enabled
-      ({{ matrix_homeserver_enabled_implementations_list|join(', ') }}).
-
-      If you have more than 1 implementation enabled, you can disable the unnecessary implementations by adding `matrix_IMPLEMENTATION_enabled: false` to your vars.yml file.
-
-      If you have 0 implementations enabled, you can enable one by adding `matrix_IMPLEMENTATION_enabled: false` to your vars.yml file (e.g. `matrix_dendrite_enabled: true`).
-  when: "matrix_homeserver_enabled_implementations_list|length != 1"
+    msg: "You need to set a valid homeserver implementation in `matrix_homeserver_implementation`"
+  when: "matrix_homeserver_implementation not in ['synapse', 'dendrite']"
 
 # We generally support Ansible 2.7.1 and above.
 - name: Fail if running on Ansible < 2.7.1

roles/matrix-dendrite/defaults/main.yml

@@ -1,7 +1,7 @@
 # Dendrite is a second-generation Matrix homeserver currently in Beta
 # See: https://github.com/matrix-org/dendrite
 
-matrix_dendrite_enabled: false
+matrix_dendrite_enabled: true
 
 matrix_dendrite_docker_image: "{{ matrix_dendrite_docker_image_name_prefix }}matrixdotorg/dendrite-monolith:{{ matrix_dendrite_docker_image_tag }}"
 matrix_dendrite_docker_image_name_prefix: "docker.io/"