Fix testinfra for Debian

test/pytest/test_docker.py

 def test_docker_is_installed(host):
     docker = host.package("docker-ce")
     assert docker.is_installed
-    assert docker.version.startswith("5:18.09.2")
+    assert docker.version.startswith("5:18.09")
 
 
 def test_docker_running_and_enabled(host):

test/pytest/test_system.py

 def test_release_is_bionic(host):
     system_info = host.system_info
-    assert system_info.release == '18.04'
+    assert system_info.release == '9.8'