update tls template config for containerd 1.3.x

[kwmonroe]

https://bugs.launchpad.net/charm-containerd/+bug/1853653

Containerd updated the config.toml sections related to tls in 1.3.x:

containerd/cri#1227

This affects our ability to relate containerd units to docker-registry units that have tls enabled. This PR updates the config template to use the current 1.3.x syntax.

Without this:

$ sudo ./crictl --runtime-endpoint=unix:///var/run/containerd/containerd.sock pull 172.31.20.67:5000/defaultbackend-amd64:1.5
FATA[0000] pulling image failed: rpc error: code = Unknown desc = failed to pull and unpack image "172.31.20.67:5000/defaultbackend-amd64:1.5": failed to resolve reference "172.31.20.67:5000/defaultbackend-amd64:1.5": failed to do request: Head https://172.31.20.67:5000/v2/defaultbackend-amd64/manifests/1.5: remote error: tls: bad certificate

With it:

$ sudo ./crictl --runtime-endpoint=unix:///var/run/containerd/containerd.sock pull 172.31.20.67:5000/defaultbackend-amd64:1.5
Image is up to date for sha256:b5af743e598496e8ebd7a6eb3fea76a6464041581520d1c2315c95f993287303

Note 1: Afaict, containerd-1.2.x (which ships with xenial) never supported custom tls with a registry. Even with these tls bits in the config.toml of a xenial deployment, they don't appear in the runtime config:

$ sudo grep cdk /etc/containerd/config.toml
        ca_file   = "/root/cdk/ca.crt"
        cert_file = "/root/cdk/server.crt"
        key_file  = "/root/cdk/server.key"
$ sudo ./crictl --runtime-endpoint=unix:///var/run/containerd/containerd.sock info | grep cdk
$

We could introduce a new template variable based on the apt version and only render 1.3 bits when running 1.3. I didn't do that here because it seemed unnecessary since invalid config is silently ignored.

Note 2: The auths section of the config.toml has also had a syntax change; the old syntax is deprecated, but still works for 1.3.x. I opted to leave the old syntax in place to ensure any xenial (containerd 1.2.x) deployments would continue to work if upgraded to this charm.

[Cynerva]

LGTM 👍