Raise minimum required rkt version due to usage of lifecycle hooks
Raise minimum required rkt version due to usage of lifecycle hooks

Kubernetes relies on `rkt enter` to execute lifecycle hooks, however
before rkt 1.20, this command is not available with rkt stage1-fly,
which is used in nova-libvirt.

See https://github.com/coreos/rkt/pull/3377

stackanetes/stackanetes