Support removing keypairs, drop ssh_config_template

• Improvements to the keypairs logic:
  • Add a present (boolean) parameter, to allow the keypair to be absent (i.e. removed)
  • Logic to extract a public key from a private key (default)
  • Option to not populate a public key (public: false)
  • Explicitly set a public key (public: ssh-ed25519 AAAAC3NzaC1l...)
• Fixed many linting issues
• Dropped support for ssh_config_template key, use ssh_config with a template lookup instead
• Fixed/extended docs