In order to allow for a smoother UX with CRIs different than Docker, we have to
make the --cri-socket command line flag optional when just one CRI is
installed.

This change does that by doing the following:

- Introduce a new runtime function (DetectCRISocket) that will attempt to
  detect a CRI socket, or return an appropriate error.
- Default to using the above function if --cri-socket is not specified and
  CRISocket in NodeRegistrationOptions is empty.
- Stop static defaulting to DefaultCRISocket. And rename it to
  DefaultDockerCRISocket. Its use is now narrowed to "Docker or not"
  distinguishment and tests.
- Introduce AddCRISocketFlag function that adds --cri-socket flag to a flagSet.
  Use that in all commands, that support --cri-socket.
- Remove the deprecated --cri-socket-path flag from kubeadm config images pull
  and deprecate --cri-socket in kubeadm upgrade apply.
Signed-off-by: Rostislav M. Georgiev <rostislavg@vmware.com>