WIP: optional heap and permgen args for java VM

I'm working on boxen-ifying our Jenkins box. One customization we had to make was increasing the maximum heap size and permgen space for the Java VM.

I've changed `dev.jenkins.plist.erb` with some optional configs that result in the proper args to the java command.

The intention was to be able to do something akin to [this site.pp snippet](https://gist.github.com/toolbear/a2290955a42edef12d71) so, for instance, a node could have increased heap and permgen, while the default would be to leave them unspecified and using the VM default values. This, however, fails because one cannot assign to a namespaced variable outside of that namespace.

Any ideas on how to accomplish what I'm looking for? One thought is setting an environment var in `config/boxen.rb`, but that'd take effect for all nodes.