Stop adding `rd.luks.key=/etc/luks/cex.key` for CEX support in sugar/config and move logic to `rdcore rootmap`

[yasminvalim]

Created originally in Butane by @travier: coreos/butane#613

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Right now (or soon), when CEX is enabled in the boot_device sugar, the user also has to add an rd.luks.key karg using either an Ignition karg (on non-OpenShift variants), or an OpenShift/MCO-level karg (on OpenShift; those are different right now -- the MCO has its own kargs logic and doesn't yet use Ignition kargs for first boot).

This unnecessarily incurs an additional reboot (in the case of non-OpenShift variants, upfront, in the case of OpenShift variants, by the MCO). Instead, we should probably have rdcore rootmap inject this karg like it does other LUKS-related kargs.

See:

• Stabilize openshift v4.19.0 on 3.5.0 butane#605
• config/fcos/v1_7_exp/translate.go: added required kernel arguments to… butane#610
• https://coreos.github.io/ignition/migrating-configs/#luks-cex
• https://coreos.github.io/butane/upgrading-fcos/#luks-cex-support