pmbootstrap init: Ask for hostname, default: device name (#1327)

* Save "" (empty string) in the user's config as hostname if the user
  let it default to the name of the device. That way, when the device
  gets changed, the user won't get the old device's name as hostname
  by accident.
* Add a test case

test/test_questions.py

@@ -219,3 +219,28 @@ def test_questions_build_options(args, monkeypatch):
     func(args, cfg)
     assert cfg == {"pmbootstrap": {"jobs": "5",
                                    "ccache_size": "2G"}}
+
+
+def test_questions_hostname(args, monkeypatch):
+    func = pmb.config.init.ask_for_hostname
+    device = "test-device"
+
+    # Valid hostname
+    fake_answers(monkeypatch, ["valid"])
+    assert func(args, device) == "valid"
+
+    # Hostname too long ("aaaaa...")
+    fake_answers(monkeypatch, ["a" * 64, "a" * 63])
+    assert func(args, device) == "a" * 63
+
+    # Fail the regex
+    fake_answers(monkeypatch, ["$invalid", "valid"])
+    assert func(args, device) == "valid"
+
+    # Begins or ends with minus
+    fake_answers(monkeypatch, ["-invalid", "invalid-", "valid"])
+    assert func(args, device) == "valid"
+
+    # Device name: empty string
+    fake_answers(monkeypatch, [device])
+    assert func(args, device) == ""