Warn if underscore character is used in hostname (#2079)

* Prevent underscore character being used in 'name'.

* Restrict underscores in hostnames, not all names.

* Use hostname validator for node name.

* Allow underscore in hostname but warn once.

* Add renaming instructions link to warning.

* Point underscore warning to FAQ section

Co-authored-by: Otto Winter <otto@otto-winter.com>

Co-authored-by: Otto Winter <otto@otto-winter.com>

tests/unit_tests/test_config_validation.py

@@ -40,6 +40,28 @@ def test_valid_name__invalid(value):
         config_validation.valid_name(value)
 
 
+@pytest.mark.parametrize("value", ("foo", "bar123", "foo-bar"))
+def test_hostname__valid(value):
+    actual = config_validation.hostname(value)
+
+    assert actual == value
+
+
+@pytest.mark.parametrize("value", ("foo bar", "foobar ", "foo#bar"))
+def test_hostname__invalid(value):
+    with pytest.raises(Invalid):
+        config_validation.hostname(value)
+
+
+def test_hostname__warning(caplog):
+    actual = config_validation.hostname("foo_bar")
+    assert actual == "foo_bar"
+    assert (
+        "Using the '_' (underscore) character in the hostname is discouraged"
+        in caplog.text
+    )
+
+
 @given(one_of(integers(), text()))
 def test_string__valid(value):
     actual = config_validation.string(value)