conftest: make pytest-html optional

We do not need pytest-html for normal functionality. It is just nice
plugin we can use to generate human readable report to go alongside with
pytest native xml and log output.
We also support only pytest-html with minimal version 2.1.0 and thus all
this is included only if pytest-html is at least of that version.

The plugin is removed by this from requirements.txt as this file lists
necessary Python packages and this is now only optional.

conftest.py

+import pkg_resources
 
-def pytest_html_report_title(report):
-    report.title = "NSFarm tests report"
+
+def pytest_configure(config):
+    html_plugin = config.pluginmanager.getplugin("html")
+    if html_plugin is not None and \
+            pkg_resources.parse_version(html_plugin.__version__) >= pkg_resources.parse_version("2.1.0"):
+        config.pluginmanager.register(HTMLReport())
+
+
+class HTMLReport:
+    """Hooks for optional Pytest HTML plugin.
+    (Pytest fails in case there is hooks with unknown handler. This way we include it only if we have pytest-html.)
+    """
+
+    def pytest_html_report_title(self, report):
+        report.title = "NSFarm tests report"

requirements.txt

 pytest>=5.0
 pytest-lazy-fixture>=0.5
-pytest-html>=2.1.0
 pexpect
 pyserial
 pylxd