Allow vagrant directory location to be configurable

In the case were vagrant is not invoked directly from the repository,
but from another location, and the Vagrantfile is "included" into
another, we need to be able to specify where the location of the vagrant
directory is, as of now it's hardcoded relative to the Vagrantfile
location. This commit fix it.

Vagrantfile

@@ -77,6 +77,8 @@ $libvirt_nested ||= false
 $ansible_verbosity ||= false
 $ansible_tags ||= ENV['VAGRANT_ANSIBLE_TAGS'] || ""
 
+$vagrant_dir ||= File.join(File.dirname(__FILE__), ".vagrant")
+
 $playbook ||= "cluster.yml"
 
 host_vars = {}
@@ -96,7 +98,7 @@ $inventory = File.absolute_path($inventory, File.dirname(__FILE__))
 # if $inventory has a hosts.ini file use it, otherwise copy over
 # vars etc to where vagrant expects dynamic inventory to be
 if ! File.exist?(File.join(File.dirname($inventory), "hosts.ini"))
-  $vagrant_ansible = File.join(File.dirname(__FILE__), ".vagrant", "provisioners", "ansible")
+  $vagrant_ansible = File.join(File.absolute_path($vagrant_dir), "provisioners", "ansible")
   FileUtils.mkdir_p($vagrant_ansible) if ! File.exist?($vagrant_ansible)
   $vagrant_inventory = File.join($vagrant_ansible,"inventory")
   FileUtils.rm_f($vagrant_inventory)