Remove duplicate SSL hostnames

Ref dokku/dokku-letsencrypt#76

Certificates provided by Let's Encrypt can have the domain provided in
the Subject Name field listed again in the Subject Alt Name fields,
leading to the same domain getting listed twice by get_ssl_hostnames()
that in turn leads to a duplicate server_name field in the nginx
configuration.

This patch adds a sort -u to ensure that every SSL hostname will only be
listed once.

plugins/certs/functions

@@ -26,6 +26,6 @@ get_ssl_hostnames() {
   else
     local SSL_HOSTNAMES=$SSL_HOSTNAME
   fi
-  echo -e "$SSL_HOSTNAMES"
+  echo -e "$SSL_HOSTNAMES" | sort -u
   return 0
 }