cli: add maintenance subcommands

[lucab]

For operational maintenance and introspection, airlock should grow some CLI subcommands.

Those are mostly meant for human consumption, in order to inspect state in etcd3 and act on it.

The idea is that the administrator can kubectl exec into the container and run maintenance commands there. Those commands requires access to the configuration, which is already available inside the container.

Here below is an initial list of tasks I'd like to be able to perform via CLI.

• [ ] lock a reboot slot (by group+uuid)
• [ ] unlock a reboot slot (by group+uuid)
• [ ] show current slots usage (optionally, filtered by group)
• [ ] list configured groups and their configured slots (optionally, filtered by group)

[lucab]

I don't want to spend too much time at this point, so I'll just put together some commands under an explicit experimental ex subcommand for now.

I'll keep this ticket open to later re-arrange in an UX-meaningful way.