base: Factor out SSH host, user cert roles
Moving the SSH host and user certificate configuration roles out of `base.yml` into their own playbooks. This will make it easier to deploy them separately, and target different sets of hosts. The main driver for this change is the OVH VPS; being external, it cannot communicate with SSHCA and thus cannot have a signed host certificate. As such, we do not want to try to configure the SSHCA client on it at all.
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