Allow nodepool for using in-cluster configs

When running within a kubernetes cluster, it's easier to configure a
service account and mounting it on the nodepool POD than creating a
kubeconfig file and making it available to nodepool.

For this reason, this commit adds the ability to load configs from the
in-cluster service account paths. It does this as a fallback when the
kubeconfig path doesn't exist.

This commit also makes `context` a non required configuration option,
since it's not needed when a service account is used.

Change-Id: I7762940993c185c17d7468df72dff22e99d7f8c2

nodepool/driver/kubernetes/config.py

@@ -109,7 +109,7 @@ class KubernetesProviderConfig(ProviderConfig):
 
         provider = {
             v.Required('pools'): [pool],
-            v.Required('context'): str,
+            'context': str,
             'launch-retries': int,
         }
 

nodepool/driver/kubernetes/provider.py

@@ -49,7 +49,13 @@ class KubernetesProvider(Provider):
             self.namespace_names.add(pool.name)
 
     def _get_client(self, context):
-        conf = config.new_client_from_config(context=context)
+        try:
+            conf = config.new_client_from_config(context=context)
+        except FileNotFoundError:
+            self.log.debug("Kubernetes config file not found, attempting "
+                           "to load in-cluster configs")
+            conf = config.load_incluster_config()
+
         return (
             k8s_client.CoreV1Api(conf),
             k8s_client.RbacAuthorizationV1beta1Api(conf))