Skip fetching of ssl_uri in context_processors if redundant

I noticed that the commit - https://github.com/richleland/django-cumulus/commit/f952f692919709881c0beedd886e8e10be6900af introduced a setting 'CONTAINER_URI' which can be specified to skip fetching the container uri from the service. However the call to fetch ssl_uri is still made. 

This commits adds a further optimization to the context_processors that if the cdn_url itself is https, then the fetching of public_ssl_uri is also skipped