Return correct scheme in version URLs if service
behind an SSL termination proxy.
This is done by adding a new configuration option,
secure_proxy_ssl_header, which, when defined, makes
the wsgi application take the host_url scheme from
that header. By default, when this option is not
specified, there is no difference in behavior.
The intention is to configure any ssl-decrypting
proxy to set that header, so that glance-api knows
which protocol to use in the URLs in response.
This patch is largely based on the equivalent
nova patch: https://review.openstack.org/#/c/206479.
Partial-bug: 1558683
Change-Id: I9a9c0e42a6ad3c18d197f10095958b48d5cb879a
513d717d28