Make `nbdev_new` create settings with `git_url` using `GH_HOST`

I am using GH_HOST env variable with my GitHub Enterprise hostname, then expecting the settings.ini reflect that in docs section but still seeing `git_url = https://github.com/%(user)s/%(repo)s`

I can manually change that but not sure what could be side effect of git_url settings.

```bash
### Docs ###
branch = master
custom_sidebar = False
doc_host = https://%(user)s.github.io
doc_baseurl = /%(repo)s
git_url = https://github.com/%(user)s/%(repo)s
title = %(lib_name)s

```
cc @seeM