Switch default beaker nodeset to Debian

Ubuntu is still available but the default is Debian 7

Author: Daniel Dreier

spec/acceptance/nodesets/default.yml

@@ -1,10 +1,10 @@
 HOSTS:
-  ubuntu-server-12042-x64:
+  debian-73-x64:
     roles:
       - master
-    platform: ubuntu-12.04-amd64
-    box : ubuntu-server-12042-x64-vbox4210-nocm
-    box_url : http://puppet-vagrant-boxes.puppetlabs.com/ubuntu-server-12042-x64-vbox4210-nocm.box
+    platform: debian-7-amd64
+    box : debian-73-x64-virtualbox-nocm
+    box_url : http://puppet-vagrant-boxes.puppetlabs.com/debian-73-x64-virtualbox-nocm.box
     hypervisor : vagrant
 CONFIG:
   log_level: debug