Use the boxen-git-credential for all repositories clones.

This is required since puppet run all the comands as root and the
credential config is only present on the user land.

The lack of this configuration is making clones using https to not
work since the credential is not present for the root.

Closes #286

manifests/site.pp

@@ -39,7 +39,10 @@ Repository {
   extra    => [
     '--recurse-submodules'
   ],
-  require  => Class['git']
+  require  => Class['git'],
+  config   => {
+    'credential.helper' => "${boxen::config::bindir}/boxen-git-credential"
+  }
 }
 
 Service {