Implement host registration

[fabriziopandini]

The host agent will be responsible for the host registration process (more responsibilities will be added in future).

In the first iteration of this component:

• The host agent will be a CLI program, to be run as a daemon or as a foreground process for demo purposes
• Host agent requires basic logging capabilities (log to file, log to stdout/stderr).
• We are targeting only linux/amd64, Ubuntu, without any special devices.
• We are not implementing support for auto discovery of host feature sets; host features will be manually passed to the host agent via a --label flag mapping the list of labels defined in Create the BYOHost API type #9
• host registration will only take care of creating the corresponding BYOHost object, without addressing any further requirement WRT to secure attestation.
• In order to create the corresponding BYOHost object a KubeConfig file should be provided to the host agent via the --kubeconfig flag; management/lifecycle of the identity used in the above kubeconfig is out of scope of this issue