Implement elemental-register subcommands

[anmazzotti]

Currently elemental-register supports a multitude of arguments that can be used all together, even though they may be mutually exclusive. This makes the code a bit harder to maintain, and harder to extend as well with new functionality.

It would be best to split into subcommands.

elemental-register version
elemental-register register --install
elemental-register install 
elemental-register reset --network-only
elemental-register verify

Global args: --config, --state, --debug

Note that backward compatibility should be granted when running elemental-register root command.

Subcommands:

• version Displays the version
• register Registers this machine, if --install is used, installation will take place automatically after registration.
• install Installs the OS on this machine.
• reset Resets the machine. If --network-only is used, it will only reset the network (consider alternative network reset subcommand)
• verify (NEW!) performs a simple verification (fetch the remote registration? also machineinventory authentication?) that can be used for boot assessment

I also propose to drop all other arguments that can be retrieved from the configuration file/remote registration, like emulate-tpm, no-smbios, ca-cert, auth, and so on).
--config should be enough as a global argument

--state is nice to have to be able to point to a state file without having to expose that in the MachineRegistration API, but I would also think about making the elemental-register stateless again. This can be simply achieved by dropping any emulated-tpm support, which will also improve maintenance and reduce complexity. We have alternative auth methods that can be equally used for non-production/demo purposes already.