Add new plugin: external - resolve k8s ingress and LB address with external names (#2379)

* Add new plugin: external

This plugin works in conjunction with the kubernetes plugin and exports
ingress and LB addresses as DNS records. It bypasses backend.go and
backend_lookup.go flow because it is not needed.

README, tests are implemented. The tests only exercise the unit tests,
this has not been tested in any ci.

Signed-off-by: Miek Gieben <miek@miek.nl>

* Rename to k8s_external

Signed-off-by: Miek Gieben <miek@miek.nl>

* go gen

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/k8s_external/setup_test.go

@@ -0,0 +1,48 @@
+package external
+
+import (
+	"testing"
+
+	"github.com/mholt/caddy"
+)
+
+func TestSetup(t *testing.T) {
+	tests := []struct {
+		input        string
+		shouldErr    bool
+		expectedZone string
+		expectedApex string
+	}{
+		{`k8s_external`, false, "", "dns"},
+		{`k8s_external example.org`, false, "example.org.", "dns"},
+		{`k8s_external example.org {
+			apex testdns
+}`, false, "example.org.", "testdns"},
+	}
+
+	for i, test := range tests {
+		c := caddy.NewTestController("dns", test.input)
+		e, err := parse(c)
+
+		if test.shouldErr && err == nil {
+			t.Errorf("Test %d: Expected error but found %s for input %s", i, err, test.input)
+		}
+
+		if err != nil {
+			if !test.shouldErr {
+				t.Errorf("Test %d: Expected no error but found one for input %s. Error was: %v", i, test.input, err)
+			}
+		}
+
+		if !test.shouldErr && test.expectedZone != "" {
+			if test.expectedZone != e.Zones[0] {
+				t.Errorf("Test %d, expected zone %q for input %s, got: %q", i, test.expectedZone, test.input, e.Zones[0])
+			}
+		}
+		if !test.shouldErr {
+			if test.expectedApex != e.apex {
+				t.Errorf("Test %d, expected apex %q for input %s, got: %q", i, test.expectedApex, test.input, e.apex)
+			}
+		}
+	}
+}