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-nginx-proxy/tasks/self_check_well_known.yml

@@ -7,6 +7,7 @@
         purpose: Client Discovery
         cors: true
         follow_redirects: false
+        validate_certs: "{{ matrix_nginx_proxy_self_check_validate_certificates }}"
 
 - block:
     - set_fact:
@@ -15,6 +16,7 @@
           purpose: Server Discovery
           cors: false
           follow_redirects: true
+          validate_certs: "{{ matrix_nginx_proxy_self_check_validate_certificates }}"
 
     - name: Determine domains that we require certificates for (mxisd)
       set_fact: