Add cluster-init --no-etc-hosts parameter

If --no-etc-hosts is passed in at cluster init time we set a new
parameter in the cluster's object to false, and avoid adding nodes to
the hosts file. The UpgradeConfig function is used to set the value to
True, when upgrading from an old configuration version.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>