Add configuration options

[nrutledge]

• [ ] Allow configuring (at least) the following options:
  • [ ] paths where to mount directories used to store the states of services (e.g. the cardano-node database or operator’s keys)
  • [ ] same options available for the SPO node (i.e. we would simply expose them for the end user)
  • [ ] secret keys to be pushed to the target host during the deployment
  • [ ] everything else which could differ across providers that we realize would need to be addressed specifically
• [ ] Configuration options are exposed as CLI arguments
• [ ] And as NixOS options
• [ ] Add tests for each configuration option (at least one expected to succeed and one expected to fail)