Add matrix_homeserver_implementation, tracking the current homeserver implementation

The goal is to have a single variable which tells us which homeserver
software is in use. Much simpler than having if/elif/elif checks for
variables like (`matrix_synapse_enabled` and `matrix_dendrite_enabled`, etc.)
everywhere.

roles/matrix-base/defaults/main.yml

@@ -7,6 +7,13 @@
 # Example value: example.com
 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: ''
+
 # 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 }}"

roles/matrix-base/tasks/sanity_check.yml

@@ -1,5 +1,16 @@
 ---
 
+- name: Fail if 0 or more than 1 homeserver implementations enabled
+  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"
+
 # We generally support Ansible 2.7.1 and above.
 - name: Fail if running on Ansible < 2.7.1
   fail:

roles/matrix-base/vars/main.yml

@@ -1,3 +1,3 @@
 # This will contain a list of enabled services that the playbook is managing.
 # Each component is expected to append its service name to this list.
-matrix_systemd_services_list: []
+matrix_systemd_services_list: []