Add maintenance mode for deployments

Add a maintenance command in the fab file in order to show a friendly web page when the web site is being updated. To update the site, a possible sequence of fab commands is:
```
build
push_build
maintenance:begin
supervisor_stop
deploy_web
supervisor
maintenance:end
```
Note that the `maintenance` command includes a call to `nginx_restart`.

The PR also includes an additional configuration parameter to explicit name the hosts for which http requests will automatically be re-written as https request.