Resolve issue #563 configure pytest marks

`pytest` generates spurious warnings which can be easily suppressed
by configuring `pytest` correctly.

Added relevant configuration

Author: salty-vagrant

test/conftest.py

@@ -257,3 +257,12 @@ def build_image(build_failed, dockerfile, image, image_path):
         thread.join()
     if build_failed.is_set():
         raise RuntimeError("One or more docker build failed")
+
+    config.addinivalue_line(
+        "markers",
+        "testinfra_hosts(host_selector): mark test to run on selected hosts"
+    )
+    config.addinivalue_line(
+        "markers",
+        "destructive: mark test as destructive"
+    )