crontab: ensure the script is run with shell

Some distros does not provide a default shell for Ceilometer user.
We can run the crontab by force shell usage and avoid running
issues.

Closes-bug: #1400523
Change-Id: I3a4d5c958a4620335d062baecfc362fd758768b6

Author: Emilien Macchi

manifests/expirer.pp

@@ -61,7 +61,7 @@
 
   cron { 'ceilometer-expirer':
     command     => $ceilometer::params::expirer_command,
-    environment => 'PATH=/bin:/usr/bin:/usr/sbin',
+    environment => 'PATH=/bin:/usr/bin:/usr/sbin SHELL=/bin/sh',
     user        => 'ceilometer',
     minute      => $minute,
     hour        => $hour,

spec/classes/ceilometer_expirer_spec.rb

@@ -44,7 +44,7 @@
     it 'configures a cron' do
       should contain_cron('ceilometer-expirer').with(
         :command     => 'ceilometer-expirer',
-        :environment => 'PATH=/bin:/usr/bin:/usr/sbin',
+        :environment => 'PATH=/bin:/usr/bin:/usr/sbin SHELL=/bin/sh',
         :user        => 'ceilometer',
         :minute      => 1,
         :hour        => 0,