Update GitFS Backend Walkthrough to include GitFS over SSH 

A small description on how to configure GitFS over SSH transport.
Make pkg.install respect __env__

This fixes #5209.
Merge pull request #5247 from terminalmage/issue5209

Make pkg.install respect __env__
Merge pull request #5244 from valentinbud/develop

Update GitFS Backend Walkthrough to include GitFS over SSH
Add docstring to interface()
Merge pull request #5251 from UtahDave/new_win_network

Add docstring to interface()
be consistent about .override use

disabling of service is correct, but checking whether
it is disabled is not

upstart service is disabled by creating {service-name}.override file
and adding a 'manual' stanza there [1]

[1] http://upstart.ubuntu.com/cookbook/#disabling-a-job-from-automatically-starting
Merge pull request #5252 from kvbik/disabling-upstart-job

be consistent about .override use