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

plugin/federation/kubernetes_api_test.go

@@ -9,6 +9,7 @@ import (
 
 type APIConnFederationTest struct{}
 
+func (APIConnFederationTest) HasSynced() bool                        { return true }
 func (APIConnFederationTest) Run()                                   { return }
 func (APIConnFederationTest) Stop() error                            { return nil }
 func (APIConnFederationTest) SvcIndexReverse(string) []*api.Service  { return nil }