This lets us use the new shared cluster tags, for shared networking
objects - in particular subnets.
We continue to add the existing tags also, for compatability. When we
add direct management of shared networks, we will likely address that.
One commit from the always incredible @kris-nova was incorporated here, it was to
check for tags in `kutil/delete_cluster.go`. She was a major driver and instrumental in getting
this to where we aere now!
* Zones are now subnets
* Utility subnet is no longer part of Zone
* Bastion InstanceGroup type added instead
* Etcd clusters defined in terms of InstanceGroups, not zones
* AdminAccess split into SSHAccess & APIAccess
* Dropped unused Multizone flag