makebuildserver: allow a list/tuple for baseboxurl to support local copy

config.vm.box_url can be a list/tuple of URLs, which is useful to specific
a locally cached copy.  This is needed on slow connections, so that if it
fails, the download of jessie32.box does not have to start from the
beginning of the file again.

makebuildserver

@@ -164,6 +164,11 @@ for f, src, shasum in cachefiles:
 
     wanted.append(f)
 
+# allow specifying a list/tuple that includes cached local copy
+if type(config['baseboxurl']) in (list, tuple) or config['baseboxurl'][0] in ('(', '['):
+    baseboxurl = config['baseboxurl']
+else:
+    baseboxurl = '"{0}"'.format(config['baseboxurl'])
 
 # Generate an appropriate Vagrantfile for the buildserver, based on our
 # settings...
@@ -178,7 +183,7 @@ Vagrant.configure("2") do |config|
   end
 
   config.vm.box = "{0}"
-  config.vm.box_url = "{1}"
+  config.vm.box_url = {1}
 
   config.vm.provider "virtualbox" do |v|
     v.customize ["modifyvm", :id, "--memory", "{2}"]
@@ -187,7 +192,7 @@ Vagrant.configure("2") do |config|
 
   config.vm.provision :shell, :path => "fixpaths.sh"
 """.format(config['basebox'],
-           config['baseboxurl'],
+           baseboxurl,
            config['memory'],
            config.get('cpus', 1))
 if 'aptproxy' in config and config['aptproxy']: