Make APT repo not mandatory

Refactored sensu::debian to be more flexible, now declaring
a sensu::debian class you can remove the repository or
setup the unstable repo, i.e.

  class { 'sensu::debian': ensure => 'absent' }
  class { 'sensu::debian': repo => 'unstable }

manifests/debian.pp

@@ -1,14 +1,24 @@
-
-class sensu::debian {
+#
+# Add APT key and repository
+#
+# == Parameters:
+#
+# $ensure::     'present' or 'absent'
+# $repo::       'main' or 'unstable'
+#
+class sensu::debian (
+  $ensure = 'present',
+  $repo   = 'main'
+  ) {
 
   sensu::apt::key { 'Sensu':
     ensure  => 'present',
     url     => 'http://repos.sensuapp.org/apt/pubkey.gpg',
   }
 
   sensu::apt::source { 'sensuapp':
-    ensure  => 'present',
-    content => 'deb http://repos.sensuapp.org/apt sensu main',
+    ensure  => $ensure,
+    content => "deb http://repos.sensuapp.org/apt sensu ${repo}",
     require => Sensu::Apt::Key['Sensu'],
   }