validate containerd config

[Kevin W Monroe]

The 'custom_registries' config option is a json array posing as a string. This is prone to error, so we should validate that the data contains the fields and data types that we expect.

Initially discussed in https://github.com/charmed-kubernetes/charm-containerd/pull/39, one example of something we need to validate is that 'ca|cert|key_file' values are b64 encoded. Another would be that each registry item has a 'url' key/value.

If we come across bad data in the config, the charm should block until the operator sets valid config.

[Adam Dyess (addyess)]

Raised PR to address:
   https://github.com/charmed-kubernetes/charm-containerd/pull/57/