Install the correct kernel "extra" modules on Ubuntu 15.10 slaves.
The old code broke if the running kernel version was different than the latest installed one (e.g. after apt-get upgrade installed a new kernel during reimaging).
# Extra kernel image for AUFS
-apt-get install -y linux-image-extra-$(uname -r)
+apt-get -y install "linux-image-extra-4.*"
-apt-get install -y apt-transport-https ca-certificates
+apt-get -y install apt-transport-https ca-certificates
apt-key adv --keyserver hkp://p80.pool.sks-keyservers.net:80 --recv-keys 58118E89F3A912897C070ADBF76221572C52609D
echo "deb https://apt.dockerproject.org/repo ubuntu-wily main" >/etc/apt/sources.list.d/docker.list
-apt-get update
+apt-get -y update
# Installing docker might fails because the docker socket is masked.
# See https://github.com/docker/docker/issues/22847
-apt-get install -y docker-engine=1.9.* \
+apt-get -y install docker-engine=1.9.* \
|| (systemctl unmask docker.socket; service docker start; apt-get install -f)
# Docker group
