Fix: GCPAuthenticator throws null exception in some cases

[hatsuyuki15]

Hi, currently GCPAuthenticator support for environment without gcloud is broken. The code will throw NullPointerException when being executed:

Exception in thread "main" java.lang.NullPointerException
	at io.kubernetes.client.util.authenticators.GCPAuthenticator.refresh(GCPAuthenticator.java:103)
	at io.kubernetes.client.util.KubeConfig.getCredentials(KubeConfig.java:220)
	at io.kubernetes.client.util.credentials.KubeconfigAuthentication.<init>(KubeconfigAuthentication.java:59)
	at io.kubernetes.client.util.ClientBuilder.kubeconfig(ClientBuilder.java:299)

// Where exception is thrown
// File: util/src/main/java/io/kubernetes/client/util/authenticators/GCPAuthenticator.java

101:  if (this.gc == null) this.gc = GoogleCredentials.getApplicationDefault().createScoped(scopes);
102:  AccessToken accessToken = gc.getAccessToken();
103:  config.put(ACCESS_TOKEN, accessToken.getTokenValue());
104:  config.put(EXPIRY, accessToken.getExpirationTime());

The reason is that gc.getAccessToken() just returns the cached access token which is null initially. You have to explicitly call gc.refreshIfExpired() before to initialize it, as documented here

The current test for this functionality is broken too. Since it mocks getAccessToken(), it will never get null exception. However, it's quite hard to mock the correct behavior of Google Oauth2 Library, so I think I'll just attempt to fix the bug first. The fix for the test might come in another PR.

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[brendandburns]

/lgtm
/approve

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This pull-request has been approved by: brendandburns, hatsuyuki15

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