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_nsexposed_test.go

@@ -4,7 +4,6 @@ package test
 
 import (
 	"testing"
-	"time"
 
 	"github.com/coredns/coredns/plugin/test"
 
@@ -42,9 +41,6 @@ func TestKubernetesNSExposed(t *testing.T) {
 	}
 	defer server.Stop()
 
-	// Work-around for timing condition that results in no-data being returned in test environment.
-	time.Sleep(3 * time.Second)
-
 	for _, tc := range dnsTestCasesAllNSExposed {
 
 		c := new(dns.Client)