[apt] Only grep for last 8 characters as this is what apt-key list returns (DataDog#374)

* Only grep for last 8 characters as this is what apt-key list returns

* [ubuntu] fix key substring extraction-interpolation

[key] get last eight characters with regex subst

Author: truthbk

manifests/ubuntu/install_key.pp

@@ -13,8 +13,10 @@
 #
 #
 define datadog_agent::ubuntu::install_key() {
+  $shortkey =  regsubst($name, '.*(.{8})$', '\1')
+
   exec { "key ${name}":
     command => "/usr/bin/apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys ${name}",
-    unless  => "/usr/bin/apt-key list | grep ${name} | grep expires",
+    unless  => "/usr/bin/apt-key list | grep ${shortkey} | grep expires",
   }
 }