New middleware to handle SSL termination proxies

The Heat API doesn't behave properly if it is behind an SSL termination
proxy. If this is the case, the HTTP redirections and the links
returned in the REST response bodies are build using http protocol
instead of https.

To handle this situation, a new middleware was added. The purpose of the
SSLMiddleware is to update the wsgi.url_scheme environment variable of
the request with the value contained in an HTTP header that can be
configured in heat.conf (by default: 'X-Forwarded-Proto')

Implements: rally-user-story US4145
Implements: rally-task TA3484

Change-Id: I418cd8cf2fb2a5fb3f630b4945084fb6eab57310
Closes-Bug: #1235555