Add variable for extra domains to get LE certs for

I felt that adding another variable was probably going to be the easiest way to do this. I may end up adding another variable to enable this feature, for consistency with some of the other things.

roles/matrix-nginx-proxy/defaults/main.yml

@@ -292,6 +292,7 @@ matrix_ssl_architecture: "amd64"
 
 # The list of domains that this role will obtain certificates for.
 matrix_ssl_domains_to_obtain_certificates_for: []
+matrix_ssl_additional_domains_to_obtain_certificates_for: []
 
 # Controls whether to obtain production or staging certificates from Let's Encrypt.
 matrix_ssl_lets_encrypt_staging: false