post-processor/vagrant: fix slice copy to actually work for override

[GH-426]

CHANGELOG.md

@@ -20,6 +20,7 @@ BUG FIXES:
 * builder/amazon/instance: Make AccessKey/SecretKey available to bundle
   command even when they come from the environment. [GH-434]
 * builder/virtualbox: F1-F12 and delete scancodes now work. [GH-425]
+* post-processor/vagrant: Override configurations properly work. [GH-426]
 * provisioner/puppet-masterless: Fix failure case when both facter vars
   are used and prevent_sudo. [GH-415]
 

post-processor/vagrant/post-processor.go

@@ -77,7 +77,7 @@ func (p *PostProcessor) Configure(raws ...interface{}) error {
 		}
 
 		// Create the proper list of configurations
-		ppConfigs := make([]interface{}, 0, len(p.rawConfigs)+1)
+		ppConfigs := make([]interface{}, len(p.rawConfigs), len(p.rawConfigs)+1)
 		copy(ppConfigs, p.rawConfigs)
 		ppConfigs = append(ppConfigs, raw)