Add mtu config for warden container interface

Adding configurable MTU size for warden containers. It will be needed if you're deploy cloudfoundry on OpenStack and you're using on OpenStack OpenVSwitch with Gre tunneling.

The network is wokring instead of rubygems and github because these will block mtu's greater then 1500. Gre will add on your TCP stack 46 Bit and your MTU will grow. So you can't communicate with rubygems or github (only with Gre, not Vlan)

This configurable option will solve the problem "To many proxy resets ... " if you try to install ruby gems. 


The cf_releases PR will follow in few mins, you will find there the changed erb template config file of warden.