Support other set of common steps/features during init

[vrutkovs]

Since we might want to test systemd containers or VMs, ctf-cli init should support more params:

• init --empty - no submodules added
• init --systemd - submodules with common steps / features for machinectl added
  etc.

[thozza]

Since there is no configuration file, I'm for having init without adding any remote steps/features. These can be added using some add command.

[vrutkovs]

#39 mostly resolves it. We might want to add some handy shortcuts, e.g. current common-steps and common-features should be renamed and can be automagically added via init --docker