Use environment variables when error is strict

** Summary **

Fix minor bug in readConfig function when cheking the error of
unmarshalSemiStrictly.

** Motivation **

This will allow us to use a configuration that has an unmarshalling
strict error, i.e. when the configuration have unknown fields, and
the validate-config and validate-config-strict command-line flags
are both false.

** Test plan **

Automated testing.

** Rollout/monitoring/revert plan **

None

config_parse.go

@@ -135,7 +135,7 @@ func readConfig(r io.Reader) (Config, error) {
 	}
 	unmarshalErr := unmarshalSemiStrictly(bts, &c)
 	if unmarshalErr != nil {
-		if _, ok := err.(*UnknownConfigKeys); !ok {
+		if _, ok := unmarshalErr.(*UnknownConfigKeys); !ok {
 			return c, unmarshalErr
 		}
 	}

config_test.go

@@ -37,6 +37,18 @@ func TestReadBadConfig(t *testing.T) {
 	assert.Equal(t, c, Config{}, "Parsing invalid config file should return zero struct")
 }
 
+func TestReadBadConfig_EnvVarsAreNotRead(t *testing.T) {
+	os.Setenv("VENEUR_HOSTNAME", "cux")
+	defer os.Unsetenv("VENEUR_HOSTNAME")
+
+	const exampleConfig = `--- api_hostname: :bad`
+	r := strings.NewReader(exampleConfig)
+	c, err := readConfig(r)
+
+	assert.NotNil(t, err, "Should have encountered parsing error when reading invalid config file")
+	assert.Equal(t, c, Config{}, "Parsing invalid config file should return zero struct")
+}
+
 func TestReadUnknownKeysConfig(t *testing.T) {
 	const config = `---
 no_such_key: 1
@@ -51,6 +63,23 @@ hostname: foobar
 	assert.Equal(t, "foobar", c.Hostname)
 }
 
+func TestReadUnknownKeysConfig_EnvVarsAreParsed(t *testing.T) {
+	os.Setenv("VENEUR_HOSTNAME", "cux")
+	defer os.Unsetenv("VENEUR_HOSTNAME")
+
+	const config = `---
+no_such_key: 1
+hostname: foobar
+`
+	r := strings.NewReader(config)
+	c, err := readConfig(r)
+	assert.Error(t, err)
+	_, ok := err.(*UnknownConfigKeys)
+	t.Log(err)
+	assert.True(t, ok, "Returned error should indicate a strictness error")
+	assert.Equal(t, "cux", c.Hostname)
+}
+
 func TestReadUnknownKeysProxyConfig(t *testing.T) {
 	const config = `---
 no_such_key: 1