Make vars.yml snapshotting optional and more configurable

Certain people organize their inventory in a different way
and we'd like to accommodate them.

Related to #542 (Github Issue).

roles/matrix-base/defaults/main.yml

@@ -59,6 +59,12 @@ matrix_riot_jitsi_preferredDomain: ''
 # The Docker network that all services would be put into
 matrix_docker_network: "matrix"
 
+# Controls whether we'll preserve the vars.yml file on the Matrix server.
+# If you have a differently organized inventory, you may wish to disable this feature,
+# or to repoint `matrix_vars_yml_snapshotting_src` to the file you'd like to preserve.
+matrix_vars_yml_snapshotting_enabled: true
+matrix_vars_yml_snapshotting_src: "{{ inventory_dir }}/host_vars/{{ inventory_hostname }}/vars.yml"
+
 # Controls whether a `/.well-known/matrix/server` file is generated and used at all.
 #
 # If you wish to rely on DNS SRV records only, you can disable this.

roles/matrix-base/tasks/setup_matrix_base.yml

@@ -12,11 +12,12 @@
 
 - name: Preserve vars.yml on the server for easily restoring if it gets lost later on
   copy:
-    src: "{{ inventory_dir }}/host_vars/{{ inventory_hostname }}/vars.yml"
+    src: "{{ matrix_vars_yml_snapshotting_src }}"
     dest: "{{ matrix_base_data_path }}/vars.yml"
     owner: "{{ matrix_user_username }}"
     group: "{{ matrix_user_groupname }}"
     mode: '0660'
+  when: "matrix_vars_yml_snapshotting_enabled|bool"
 
 # `docker_network` doesn't work as expected when the given network
 # is a substring of a network that already exists.