kubernetes: add optional support for a kubeadm.yaml
kubernetes: add optional support for a kubeadm.yaml

Unfortunately some options (such as enabling dynamic registration of
initializers) can only be enabled by a `--config foo.yaml` argument.
Furthermore some command-line options (such as the kubernetes version)
cannot be used in combination with the config file.

This patch checks for a supplied  /etc/kubeadm/kubeadm.yaml and uses
it if it exists, otherwise it falls back to the original command-line.
Note it is safe to use the `--skip-*` options in combination with the
`--config` option.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

mor1/linuxkit,zx2c4/linuxkit,yankcrime/linuxkit,t-koulouris/linuxkit,deitch/linuxkit,mor1/linuxkit,davefreitag/linuxkit,justincormack/linuxkit,zx2c4/linuxkit,thebsdbox/linuxkit,djs55/linuxkit,davefreitag/linuxkit,zx2c4/linuxkit,thebsdbox/linuxkit,konstruktoid/linuxkit,djs55/linuxkit,ndauten/linuxkit,yankcrime/linuxkit,djs55/linuxkit,t-koulouris/linuxkit,YuPengZTE/linuxkit,linuxkit/linuxkit,YuPengZTE/linuxkit,furious-luke/linuxkit,davefreitag/linuxkit,rn/linuxkit,furious-luke/linuxkit,deitch/linuxkit,deitch/linuxkit,konstruktoid/linuxkit,konstruktoid/linuxkit,YuPengZTE/linuxkit,linuxkit/linuxkit,JohnnyLeone/linuxkit,konstruktoid/linuxkit,yankcrime/linuxkit,justincormack/linuxkit,t-koulouris/linuxkit,ndauten/linuxkit,davefreitag/linuxkit,thebsdbox/linuxkit,JohnnyLeone/linuxkit,djs55/linuxkit,t-koulouris/linuxkit,eyz/linuxkit,linuxkit/linuxkit,furious-luke/linuxkit,zx2c4/linuxkit,mor1/linuxkit,t-koulouris/linuxkit,eyz/linuxkit,justincormack/linuxkit,JohnnyLeone/linuxkit,ndauten/linuxkit,justincormack/linuxkit,JohnnyLeone/linuxkit,rn/linuxkit,mor1/linuxkit,deitch/linuxkit,ndauten/linuxkit,thebsdbox/linuxkit,eyz/linuxkit,JohnnyLeone/linuxkit,justincormack/linuxkit,justincormack/linuxkit,yankcrime/linuxkit,thebsdbox/linuxkit,furious-luke/linuxkit,ndauten/linuxkit,yankcrime/linuxkit,mor1/linuxkit,djs55/linuxkit,davefreitag/linuxkit,furious-luke/linuxkit,eyz/linuxkit,linuxkit/linuxkit,linuxkit/linuxkit,zx2c4/linuxkit,rn/linuxkit,thebsdbox/linuxkit,YuPengZTE/linuxkit,yankcrime/linuxkit,konstruktoid/linuxkit,deitch/linuxkit,YuPengZTE/linuxkit,rn/linuxkit,eyz/linuxkit,ndauten/linuxkit,eyz/linuxkit,rn/linuxkit,YuPengZTE/linuxkit,konstruktoid/linuxkit