Wrong secure_proxy_ssl_header value in nova.conf.j2 template

The value set for "secure_proxy_ssl_header" should be
"HTTP_X_FORWARDED_PROTO" and not "X-Forwarded-Proto".

Change-Id: I7f4cc4871164ca9096a190101c179daa41e1ae9a
Closes-Bug: #1719325

ansible/roles/nova/templates/nova.conf.j2

@@ -216,7 +216,7 @@ debug = {{ nova_logging_debug }}
 [wsgi]
 api_paste_config = /etc/nova/api-paste.ini
 {% if kolla_enable_tls_external | bool %}
-secure_proxy_ssl_header = X-Forwarded-Proto
+secure_proxy_ssl_header = HTTP_X_FORWARDED_PROTO
 {% endif %}
 
 [scheduler]