Move around some packages (#658)

* Move tracing exporter registration

This doesn't belong in the library and should be configured by the
consumer of the opencensus package.

* Rename `vkublet` package to `node`

`vkubelet` does not convey any information to the consumers of the
package.
Really it would be nice to move this package to the root of the repo,
but then you wind up with... interesting... import semantics due to the
repo name... and after thinking about it some, a subpackage is really
not so bad as long as it has a name that convey's some information.

`node` was chosen since this package deals with all the semantics of
operating a node in Kubernetes.

virtual-kubelet/commands/root/flag.go

@@ -22,7 +22,6 @@ import (
 
 	"github.com/pkg/errors"
 	"github.com/spf13/pflag"
-	"github.com/virtual-kubelet/virtual-kubelet/trace/opencensus"
 	"k8s.io/klog"
 )
 
@@ -74,7 +73,7 @@ func installFlags(flags *pflag.FlagSet, c *Opts) {
 	flags.IntVar(&c.PodSyncWorkers, "pod-sync-workers", c.PodSyncWorkers, `set the number of pod synchronization workers`)
 	flags.BoolVar(&c.EnableNodeLease, "enable-node-lease", c.EnableNodeLease, `use node leases (1.13) for node heartbeats`)
 
-	flags.StringSliceVar(&c.TraceExporters, "trace-exporter", c.TraceExporters, fmt.Sprintf("sets the tracing exporter to use, available exporters: %s", opencensus.AvailableTraceExporters()))
+	flags.StringSliceVar(&c.TraceExporters, "trace-exporter", c.TraceExporters, fmt.Sprintf("sets the tracing exporter to use, available exporters: %s", AvailableTraceExporters()))
 	flags.StringVar(&c.TraceConfig.ServiceName, "trace-service-name", c.TraceConfig.ServiceName, "sets the name of the service used to register with the trace exporter")
 	flags.Var(mapVar(c.TraceConfig.Tags), "trace-tag", "add tags to include with traces in key=value form")
 	flags.StringVar(&c.TraceSampleRate, "trace-sample-rate", c.TraceSampleRate, "set probability of tracing samples")