fcos.md

* set selinux type t_etc if selinux state is enforcing

* workaround with update repo is no longer needed
remove comments about failing playbook

* grubby is not available in distros using ostree

* remove docker support because removed in fcos
update install script example with live rootfs

* do not call grubby on ostree based distro

* update docs enabling containerd on fedora coreos