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/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: []