These are based on recommendation from
[staticcheck](http://staticcheck.io/).
- Remove unused struct fields
- Remove unused function
- Remove unused variables
- Remove unused constants.
- Miscellaneous cleanups
systemd is the recommended driver as per the setup of running
the kubelet using systemd as the init system. Add a preflight
check that throws a warning if this isn't the case.
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.
