Merge ~hloeung/influxdb-charm:master into influxdb-charm:master

[Haw Loeung]

Install influxdb-client package if required

Upstream packaged InfluxDB includes the influx CLI shell tool in the
'influxdb' package. Unfortunately, Debian/Ubuntu does not and splits
it out into the 'influxdb-client' package. Let's work around this
here.

I should note that a previous change landed which favors the config shipped out by packages over the charm.

diff --git a/reactive/influxdb.py b/reactive/influxdb.py
index a8394de..e94869e 100644
--- a/reactive/influxdb.py
+++ b/reactive/influxdb.py
@@ -2,11 +2,13 @@ from charmhelpers.contrib.charmsupport import nrpe
 from charmhelpers.core import hookenv
 from charmhelpers.core import host
 from charmhelpers.core.templating import render
+from charmhelpers.fetch import apt_install
 from charms.reactive import hook, set_state, remove_state, when, when_all, when_not
 
 import re
 import os
 import shutil
+import subprocess
 import yaml
 
 CRONFILE = '/etc/cron.daily/influxdb-charm-backup'
@@ -34,6 +36,12 @@ def install_influx():
     hookenv.status_set('active', '')
     set_state('influxdb.configured')
     set_state('config.changed')
+    # This is a hack because the CLI shell tool is packaged differently in
+    # upstream and Debian/Ubuntu. Upstream has no 'influxdb-client' package.
+    try:
+        subprocess.check_call(['influx', '--version'], stdin=subprocess.PIPE, stdout=subprocess.PIPE)
+    except OSError:
+        apt_install('influxdb-client')
 
 
 def apply_configs(fh, config):