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 }}"