roles/apache: Support setting ServerTokens

The `apache_server_tokens` variable can now be set, which controls the
value of the `ServerTokens` directive. If the variable is set, the
`ServerTokens` directive will be added to the `00-servername.conf` file.

roles/apache/templates/servername.httpd.conf.j2

@@ -10,3 +10,7 @@ ServerName {{ apache_server_name }}
 {% else %}
 #ServerName www.example.com:80
 {% endif %}
+{% if apache_server_tokens is defined %}
+
+ServerTokens {{ apache_server_tokens }}
+{% endif %}