[UX] Run Firecracker without API

[andreeaflorescu]

Once we have the possibility to configure Firecracker via a configuration file (#923) we can also look into the possibility to disable/compile out the API server.

As of now, there are two available options:

• use conditional compilation to compile out the api_server
• soft disable of the API (aka do not create the API thread or initialize the API server)

Each of these options come with trade-offs and before decided which way to go, we should get more input from people wanting to use Firecracker without the API.

For additional information you can check the initial issue: #923