Merge branch '178-fix-pytestunknownmarkwarning' into 'master'

Resolve "Fix PytestUnknownMarkWarning"

Closes #178

See merge request openappstack/bootstrap!102

test/pytest.ini

+[pytest]
+# Register custom markers, see https://docs.pytest.org/en/latest/mark.html
+markers =
+    certs: Run tests related to TLS certificates
+    testinfra: Run testinfra tests (test OS/package versions etc)