Update k8s client-go to v6.0.0 (#1340)

* Update k8s client-go to v6.0.0

This fix updates k8s client-go to v6.0.0 as CoreDNS is supported
in 1.9 and v6.0.0 is the recommended version.

There are quite some massive changes that need to be made:
1. k8s.io/client-go/pkg/api/v1 has been changed to k8s.io/api/v1 (repo changed from `client-go` to `api`)
2. kubernetes.Clientset adds one extra layer, so that `kubernetes.Clientset.Services()` and like has been changed to `kubernetes.Clientset.CoreV1().Services()`

Also, we have to stick with specific commits of `k8s.io/apimachinery` and the newly introduced `k8s.io/api`
because go dep still could not figure out the right version to fetch.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Update vendor with `dep ensure --update` and `dep prune`

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

vendor/k8s.io/api/OWNERS

@@ -0,0 +1,51 @@
+approvers:
+- erictune
+- lavalamp
+- smarterclayton
+- thockin
+- liggitt
+# - bgrant0607 # manual escalations only
+reviewers:
+- brendandburns
+- caesarxuchao
+- davidopp
+- dchen1107
+- deads2k
+- derekwaynecarr
+- dims
+- eparis
+- erictune
+- errordeveloper
+- feiskyer
+- gmarek
+- janetkuo
+- jbeda
+- jsafrane
+- jszczepkowski
+- justinsb
+- krousey
+- lavalamp
+- liggitt
+- luxas
+- madhusudancs
+- mikedanese
+- mwielgus
+- ncdc
+- nikhiljindal
+- piosz
+- pmorie
+- pwittrock
+- roberthbailey
+- rootfs
+- saad-ali
+- smarterclayton
+- soltysh
+- sttts
+- tallclair
+- thockin
+- timothysc
+- vishh
+- wojtek-t
+- yifan-gu
+- yujuhong
+- zmerlynn