Drop the idea of grouping step plugins

[psss]

In the initial design of tmt there was the idea to group related/similar plugins, for example:

• have virtual.testcloud and virtual.libvirt for provisioning virtual machines
• or container.podman and container.docker for two different ways to provision a container

As we've agreed on the hacking session, there does not seem to be a clear use case for this. Moreover, in order to make this working, all plugins in the same group would have to provide identical set of options, which does not seem realistic.

So the proposal is to get rid of virtual.testcloud in the current tmt code and use virtual only. For backward compatibility, the testcloud plugin should support the old name as well.