[prepare-checkbox-sanity] config_filename is not work with checkbox-remote

[jeremyszu]

#!/usr/bin/env checkbox-cli
[launcher]
launcher_version = 1

[config]
config_filename = /home/ubuntu/.cache/prepare-checkbox/checkbox.conf

[test plan]
unit = com.canonical.certification::jeremy-plan
forced = yes

[test selection]
forced = yes
exclude =

[ui]
type = silent

[daemon]
normal_user = ubuntu

---

checkbox-cli --verbose remote -u ubuntu 192.168.101.102 ./generic_launcher

---

The checkbox will not check the
```
[config]
config_filename
```
in slave machines if using checkbox-remote architecture.

The environments are not pass to slave side.
A possible way is to change to use [environment].

[jeremyszu (os369510)]

Confirmed it's fixed.