Add matrix_nginx_proxy_ssl_config

A new variable called `matrix_nginx_proxy_ssl_config` is created for
configuring how the nginx proxy configures SSL. Also a new configuration
validation option and other auxiliary variables are created.

A new variable configuration called `matrix_nginx_proxy_ssl_config` is
created. This allow to set the SSL configuration easily using the
default options proposed by Mozilla. The default configuration is set to
"Intermediate", removing the weak ciphers used in the old
configurations.

The new variable can also be set to "Custom" for a more granular control.
This allows to set another three variables called:

- `matrix_nginx_proxy_ssl_protocols`,
- `matrix_nginx_proxy_ssl_prefer_server_ciphers`
- `matrix_nginx_proxy_ssl_ciphers`

Also a new task is added to validate the SSL configuration variable.

roles/matrix-nginx-proxy/tasks/validate_config.yml

@@ -18,3 +18,9 @@
     msg: >-
       `matrix_ssl_retrieval_method` needs to be set to a known value.
   when: "matrix_ssl_retrieval_method not in ['lets-encrypt', 'self-signed', 'manually-managed', 'none']"
+
+- name: Fail on unknown matrix_nginx_proxy_ssl_config
+  fail:
+    msg: >-
+      `matrix_nginx_proxy_ssl_config` needs to be set to a known value.
+  when: "matrix_nginx_proxy_ssl_config not in ['Custom', 'Old', 'Intermediate', 'Modern']"