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.
Dustin