default channel is etcd2, but kube-apiserver requires etcd3

[Jason Hobbs]

With the default channel settings, we get stuck with kubernetes-master in blocked:
http://paste.ubuntu.com/p/8nY9Zr7qQ3/

Here is the bundle:
http://paste.ubuntu.com/p/Ps4mf8ZXCg/

Here's why kube-apiserver doesn't start:
Apr 23 15:17:20 juju-288ffb-kubernetes-10 kube-apiserver.daemon[13765]: Error: --storage-backend invalid, allowed values: etcd3. If not specified, it will default to 'etcd3'

The workaround is to use this channel setting for etcd:
https://github.com/charmed-kubernetes/bundle/blob/c2a8b42e4d5466e7ceb508c29b0365b4330532b1/fragments/k8s/cdk/bundle.yaml#L43-L44

<Cynerva> the etcd charm installs etcd2 by default. which is silly, but we haven't bumped it 'cause it causes problems with upgrades if we do it the trivial way (it upgrades etcd2 users to etcd3 before they're ready for it)

[Cory Johns (johnsca)]

This was fixed in https://bugs.launchpad.net/charm-etcd/+bug/1852676 (https://github.com/charmed-kubernetes/layer-etcd/pull/165) which was released with 1.17

[Cory Johns (johnsca)]

For clarification, the charm now installs the latest version available at the time of deploy and pins to that version's track, so major ver upgrades requires the config change.