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/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.