Acceptance tests: use avocado.utils.ssh for SSH interaction

This replaces paramiko with avocado.utils.ssh module, which is based
on a (open)ssh binary, supposedly more ubiquitous.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20190919225905.10829-1-crosa@redhat.com>
[Cleber: consolidated existing skipUnless from tests to setUp]
Signed-off-by: Cleber Rosa <crosa@redhat.com>