stoney cloud: Switch to testing branch

Login as root and execute the following to switch a stoney cloud installation from stable (releases) to testing (pre-releases):

branch="1.2-pre"

cd /usr/portage
git remote set-branches --add origin "${branch}"
git pull --rebase
git checkout "${branch}"
chown -R portage\: .

cd /var/lib/layman/foss-cloud
git remote set-branches --add origin "${branch}"
git pull --rebase
git checkout "${branch}"
chown -R portage\: .