Make self-check not validate self-signed certificates

By default, `--tags=self-check` no longer validates certificates
when `matrix_ssl_retrieval_method` is set to `self-signed`.

Besides this default, people can also enable/disable validation using the
individual role variables manually.

Fixes #124 (Github Issue)

roles/matrix-riot-web/tasks/self_check_riot_web.yml

@@ -7,6 +7,7 @@
   uri:
     url: "{{ riot_web_url_endpoint_public }}"
     follow_redirects: false
+    validate_certs: "{{ matrix_riot_web_self_check_validate_certificates }}"
   register: result_riot_web
   ignore_errors: true