support 'kubectl drain'

support equivalent of 'kubectl drain' in pykube.

$kubectl drain --help
Drain node in preparation for maintenance. 

The given node will be marked unschedulable to prevent new pods from arriving. 'drain' evicts the pods if the APIServer
supports eviction (http://kubernetes.io/docs/admin/disruptions/). Otherwise, it will use normal DELETE to delete the
pods. The 'drain' evicts or deletes all pods except mirror pods (which cannot be deleted through the API server).  If
there are DaemonSet-managed pods, drain will not proceed without --ignore-daemonsets, and regardless it will not delete
any DaemonSet-managed pods, because those pods would be immediately replaced by the DaemonSet controller, which ignores
unschedulable markings.  If there are any pods that are neither mirror pods nor managed by ReplicationController,
ReplicaSet, DaemonSet, StatefulSet or Job, then drain will not delete any pods unless you use --force.  --force will
also allow deletion to proceed if the managing resource of one or more pods is missing. 

'drain' waits for graceful termination. You should not operate on the machine until the command completes. 

When you are ready to put the node back into service, use kubectl uncordon, which will make the node schedulable again. 

! http://kubernetes.io/images/docs/kubectl_drain.svg

Examples:
  # Drain node "foo", even if there are pods not managed by a ReplicationController, ReplicaSet, Job, DaemonSet or
StatefulSet on it.
  $ kubectl drain foo --force

  # As above, but abort if there are pods not managed by a ReplicationController, ReplicaSet, Job, DaemonSet or
StatefulSet, and use a grace period of 15 minutes.
  $ kubectl drain foo --grace-period=900

Options:
      --delete-local-data=false: Continue even if there are pods using emptyDir (local data that will be deleted when
the node is drained).
      --force=false: Continue even if there are pods not managed by a ReplicationController, ReplicaSet, Job, DaemonSet
or StatefulSet.
      --grace-period=-1: Period of time in seconds given to each pod to terminate gracefully. If negative, the default
value specified in the pod will be used.
      --ignore-daemonsets=false: Ignore DaemonSet-managed pods.
      --timeout=0s: The length of time to wait before giving up, zero means infinite

Usage:
  kubectl drain NODE [options]

Use "kubectl options" for a list of global command-line options (applies to all commands).
kelproject / pykube

support 'kubectl drain' #145
