Previously, we would try to connect also to hosts that are not required
by selected test. Those hosts are likely to be offline, therefore
--mh-lazy-ssh was wrongly required.

We will add a generic connection API so other methods besides ssh
can be used. This call is used to login to the host as given user
and to test the ssh connection. It is test specific and if it is
needed it should be provided by the test framework built on
pytest-mh, not by pytest-mh itself.

This replaces current SSH connection class with generic interface.
SSHClient now implements this interface. This allows us to write
additional connectors like podman or winrm.

This is a big change that breaks backwards compatibility:

 * `MultihostHost.ssh: SSHClient`  was replaced by `MultihostHost.conn:Connection`