Refactor provider init (#360)

* Refactor provider init

This moves provider init out of vkubelet setup, instead preferring to
initialize vkubelet with a provider.

* Split API server configuration from setup.

This makes sure that configuration (which is done primarily through env
vars) is separate from actually standing up the servers.

This also makes sure to abort daemon initialization if the API servers
are not able to start.

cmd/client.go

@@ -0,0 +1,35 @@
+package cmd
+
+import (
+	"os"
+
+	"github.com/pkg/errors"
+	"k8s.io/client-go/kubernetes"
+	"k8s.io/client-go/rest"
+	"k8s.io/client-go/tools/clientcmd"
+)
+
+func newClient(configPath string) (*kubernetes.Clientset, error) {
+	var config *rest.Config
+
+	// Check if the kubeConfig file exists.
+	if _, err := os.Stat(configPath); !os.IsNotExist(err) {
+		// Get the kubeconfig from the filepath.
+		config, err = clientcmd.BuildConfigFromFlags("", configPath)
+		if err != nil {
+			return nil, errors.Wrap(err, "error building client config")
+		}
+	} else {
+		// Set to in-cluster config.
+		config, err = rest.InClusterConfig()
+		if err != nil {
+			return nil, errors.Wrap(err, "error building in cluster config")
+		}
+	}
+
+	if masterURI := os.Getenv("MASTER_URI"); masterURI != "" {
+		config.Host = masterURI
+	}
+
+	return kubernetes.NewForConfig(config)
+}