Update Kubernetes test

[pothos]

• kola/tests/kubernetes: do not require Flannel to start before Docker
  Now that flannel itself is started with Docker, we can't start Flannel
  before Docker which results in a deadlock but instead have to rely on
  Flannel to restart Docker to apply its settings.
• kola/tests/kubernetes: ensure /run/flannel exists and simplify setup
  Placing the symlink in /run/flannel/ could fail if the directory did
  not exist before. Instead of setting up a symlink, directly write the
  template to /run/flannel/ which takes care of creating the directory.
• kola/tests/kubernetes: use coreos-metadata to find the IP address
  The nodes were assumed to use coreos-cloudinit when an empty
  configuation is given and in result, set up /etc/environment.
  Use coreos-metadata (afterburn) explicitly to find the IP address
  on any cloud environment. Also, add a few more comments about the
  set up and force that no local etcd can start on the K8s controller.
• kola/tests/kubernetes: switch to 192.168.0.0/16 network
  The 10.x.0.0/16 networks overlapped with the private network 10.0.0.0/8
  used in Equinix Metal and can also overlap with Digital Ocean private
  IP addresses. The 192.168.0.0/16 network seems to be not used by AWS
  nor the internal Docker network at the moment.
• kola/tests/kubernetes: test on more platforms
  The tests were only running on GCE and by enabling it on more platforms
  we have a better chance to catch errors early.
  For VMware in our setup there are 4 tests in parallel for 12 slots and
  thus we have to limit the cluster size to 3 nodes.
• kola/tests/kubernetes: drop kubelet-wrapper test
  The kubelet-wrapper script will be removed.

Can be done in a follow-up PR:
TODO: Replace running kubelet-wrapper with Docker, remove supporting rkt as K8s runtime
TODO: Try to fix running on esx and packet platforms

How to use/testing done

Tested on AWS, DO, GCE

[t-lo]

LGTM, thank you.

[pothos]

The test should still work with rkt instead of docker, otherwise we can add ExcludeChannels: []string{"beta", "stable", "lts"}, and remove each channel when a > 2801 release comes in it.