use lowercase hostnames for node names

Usage of names containing uppercase characters returned by calls to
gethostname and getfqdn in requests to apiserver related to nodes
results in 404 errors. Node names are lowercase in K8s itself so charms
should make sure to use lowercase names well as it results in errors.

pkg/util/node/node.go has code to convert hostnames to lowercase in
GetHostname and that function is used to form node names.

cluster/juju/layers/kubernetes-worker/reactive/kubernetes_worker.py

@@ -173,7 +173,7 @@ def shutdown():
     '''
     try:
         if os.path.isfile(kubeconfig_path):
-            kubectl('delete', 'node', gethostname().lower())
+            kubectl('delete', 'node', get_node_name())
     except CalledProcessError:
         hookenv.log('Failed to unregister node.')
     service_stop('snap.kubelet.daemon')
@@ -314,7 +314,7 @@ def send_data(tls, kube_control):
     sans = [
         hookenv.unit_public_ip(),
         ingress_ip,
-        gethostname()
+        get_node_name()
     ]
 
     # Create a path safe name by removing path characters from the unit name.
@@ -1058,9 +1058,9 @@ def get_node_name():
     elif is_state('endpoint.openstack.ready'):
         cloud_provider = 'openstack'
     if cloud_provider == 'aws':
-        return getfqdn()
+        return getfqdn().lower()
     else:
-        return gethostname()
+        return gethostname().lower()
 
 
 class ApplyNodeLabelFailed(Exception):