Diffusion - default hosting options on during repository creation

Summary: ...if pertinent environment variables are set that is... Fixes T4151. This is the last piece in making repository creation somewhat easier.

Test Plan: made a new repo and noted that http serving was on r/w and ssh serving was still off, as expected for my environment configuration

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T4151

Differential Revision: https://secure.phabricator.com/D10839