plugin/kubernetes: implement HasSynced() (#1155)

* plugin/kubernetes: wait until api is ready

Wait for HasSynced before allowing startup to avoid startup race.

Also do a small refactor in findServices() to pull a check out of the
loop - only needs to be done once.

* sigh

test/kubernetes_test.go

@@ -279,8 +279,7 @@ func doIntegrationTests(t *testing.T, corefile string, testCases []test.Case) {
 	}
 	defer server.Stop()
 
-	// Work-around for timing condition that results in no-data being returned in test environment.
-	time.Sleep(3 * time.Second)
+	time.Sleep(1 * time.Second)
 
 	for _, tc := range testCases {