Fix directory /etc/nagios/nrpe.d/ issue (#492)

* Fix directory /etc/nagios/nrpe.d/  issue

Under certain deployment conditions, the charm can attempt to write to
the /etc/nagios/nrpe.d/ directory before it exists.  This directory is
created by the nrpe charm, but if the hacluster gets installed first,
then it can be triggered to attempt to set up the nrpe entries before
the directory can be created by nrpe.  This change (and the associated
charm-helpers change) ensures that the charm will delay the nrpe config
until the directory is available (and thus, the nrpe charm is fully
installed)

Related Bug: LP: #1882557

* Simplify the check so it doesn't do the write.

Co-authored-by: David Ames <david.ames@canonical.com>

charmhelpers/contrib/charmsupport/nrpe.py

@@ -18,14 +18,14 @@
 # Authors:
 #  Matthew Wedgwood <matthew.wedgwood@canonical.com>
 
-import subprocess
-import pwd
+import glob
 import grp
 import os
-import glob
-import shutil
+import pwd
 import re
 import shlex
+import shutil
+import subprocess
 import yaml
 
 from charmhelpers.core.hookenv import (
@@ -265,6 +265,11 @@ def __init__(self, hostname=None, primary=True):
                 relation_set(relation_id=rid, relation_settings={'primary': self.primary})
         self.remove_check_queue = set()
 
+    @classmethod
+    def does_nrpe_conf_dir_exist(cls):
+        """Return True if th nrpe_confdif directory exists."""
+        return os.path.isdir(cls.nrpe_confdir)
+
     def add_check(self, *args, **kwargs):
         shortname = None
         if kwargs.get('shortname') is None:
@@ -310,6 +315,12 @@ def write(self):
 
         nrpe_monitors = {}
         monitors = {"monitors": {"remote": {"nrpe": nrpe_monitors}}}
+
+        # check that the charm can write to the conf dir.  If not, then nagios
+        # probably isn't installed, and we can defer.
+        if not self.does_nrpe_conf_dir_exist():
+            return
+
         for nrpecheck in self.checks:
             nrpecheck.write(self.nagios_context, self.hostname,
                             self.nagios_servicegroups)