Add a new utility file - util/cgroupdriver.go.
Currently it only contains the public function GetCgroupDriverDocker().
The function uses 'docker info' to obtain the cgroup driver
for Docker.

On a later stage this file can contain more methods for different
CRI.

Use GetCgroupDriverDocker() in phases/kubelet/flags.go
to conditionally set the 'cgroup-driver' argument. On error
print a warning and don't set the argument value.

Add unit tests in cgroupdriver_test.go.