tests: Validation of UI access does not handle custom base path

[gdemonet]

Component: tests, ui

What happened: See #3415 (comment)

What was expected: Platforms with a changed UI base path (see #3206) should still pass our post-install test suite.

Resolution proposal (optional): From the test (

metalk8s/tests/post/steps/test_ui.py

Lines 15 to 24 in d66ec12

	@then("we can reach the UI")
	def reach_UI(host):
	ip = utils.get_grain(host, "metalk8s:control_plane_ip")
	response = requests.get(
	"https://{ip}:8443".format(ip=ip),
	verify=False,
	)
	assert response.status_code == 200, response.text

), use the expected Ingress object to derive the URL to use for accessing the UI.