add nrpe-external-master relation to kubernetes-master and kubernetes-worker

For now, the checks are very basic and only check if the systemd
services are running properly.

cluster/juju/layers/kubernetes-master/reactive/kubernetes_master.py

@@ -33,8 +33,7 @@ from charms import layer
 from charms.reactive import hook
 from charms.reactive import remove_state
 from charms.reactive import set_state
-from charms.reactive import when
-from charms.reactive import when_not
+from charms.reactive import when, when_any, when_not
 from charms.reactive.helpers import data_changed
 from charms.kubernetes.flagmanager import FlagManager
 
@@ -43,6 +42,7 @@ from charmhelpers.core import host
 from charmhelpers.core import unitdata
 from charmhelpers.core.templating import render
 from charmhelpers.fetch import apt_install
+from charmhelpers.contrib.charmsupport import nrpe
 
 
 dashboard_templates = [
@@ -489,6 +489,44 @@ def ceph_storage(ceph_admin):
     set_state('ceph-storage.configured')
 
 
+@when('nrpe-external-master.available')
+@when_not('nrpe-external-master.initial-config')
+def initial_nrpe_config(nagios=None):
+    set_state('nrpe-external-master.initial-config')
+    update_nrpe_config(nagios)
+
+
+@when('kubernetes-master.components.started')
+@when('nrpe-external-master.available')
+@when_any('config.changed.nagios_context',
+          'config.changed.nagios_servicegroups')
+def update_nrpe_config(unused=None):
+    services = ('kube-apiserver', 'kube-controller-manager', 'kube-scheduler')
+
+    hostname = nrpe.get_nagios_hostname()
+    current_unit = nrpe.get_nagios_unit_name()
+    nrpe_setup = nrpe.NRPE(hostname=hostname)
+    nrpe.add_init_service_checks(nrpe_setup, services, current_unit)
+    nrpe_setup.write()
+
+
+@when_not('nrpe-external-master.available')
+@when('nrpe-external-master.initial-config')
+def remove_nrpe_config(nagios=None):
+    remove_state('nrpe-external-master.initial-config')
+
+    # List of systemd services for which the checks will be removed
+    services = ('kube-apiserver', 'kube-controller-manager', 'kube-scheduler')
+
+    # The current nrpe-external-master interface doesn't handle a lot of logic,
+    # use the charm-helpers code for now.
+    hostname = nrpe.get_nagios_hostname()
+    nrpe_setup = nrpe.NRPE(hostname=hostname)
+
+    for service in services:
+        nrpe_setup.remove_check(shortname=service)
+
+
 def create_addon(template, context):
     '''Create an addon from a template'''
     source = 'addons/' + template