[pyramid] set waitress url_scheme to https when base-url scheme is https
See https://docs.pylonsproject.org/projects/waitress/en/latest/#using-behind-a-reverse-proxy
Since we do not configure trusted_proxy waitress ignore X-Forwarded-Proto
header and may generate http urls (pyramid request.url) when the instance is
behind a https reverse proxy. This cause cubicweb-signredrequest to not work
with since it rely on the url (including scheme).
Set url_scheme to 'https' when CubicWeb base-url scheme is https as a
workaround.