Add support for using DNS endpoint when connecting to GKE cluster #115

Add support for using DNS endpoint when connecting to GKE cluster instead of the private control plane endpoint. This functionally was added by Google couple of weeks ago

[WARNING] Failed to authenticate kubectl for cluster pj-cluster-test/europe-west1/test-cluster: E0327 10:10:29.621621 53 memcache.go:265] "Unhandled Error" err="couldn't get current server API group list: Get "https://gke-94fc1769e0c844e99793ee485d88c68c0b39.europe-west1.gke.goog/api?timeout=32s\": tls: failed to verify certificate: x509: certificate signed by unknown authority"
E0327 10:10:34.710502 53 memcache.go:265] "Unhandled Error" err="couldn't get current server API group list: Get "https://gke-94fc1769e0c844e99793ee485d88c68c0b39.europe-west1.gke.goog/api?timeout=32s\": tls: failed to verify certificate: x509: certificate signed by unknown authority"
E0327 10:10:39.806570 53 memcache.go:265] "Unhandled Error" err="couldn't get current server API group list: Get "https://gke-94fc1769e0c844e99793ee485d88c68c0b39.europe-west1.gke.goog/api?timeout=32s\": tls: failed to verify certificate: x509: certificate signed by unknown authority"
E0327 10:10:44.894223 53 memcache.go:265] "Unhandled Error" err="couldn't get current server API group list: Get "https://gke-94fc1769e0c844e99793ee485d88c68c0b39.europe-west1.gke.goog/api?timeout=32s\": tls: failed to verify certificate: x509: certificate signed by unknown authority"
E0327 10:10:49.984559 53 memcache.go:265] "Unhandled Error" err="couldn't get current server API group list: Get "https://gke-94fc1769e0c844e99793ee485d88c68c0b39.europe-west1.gke.goog/api?timeout=32s\": tls: failed to verify certificate: x509: certificate signed by unknown authority"
Unable to connect to the server: tls: failed to verify certificate: x509: certificate signed by unknown authority
[WARNING] Authentication failed for cluster pj-cluster-test/europe-west1/test-cluster