Create the external network bridge in L3 agent

* Adds code to establish the external network bridge
  for OpenVSwitch environment plugins (i.e. not NVP,
  BigSwitch or Plumgrid)
* Ensure ipv4 forward sysctl set on L3 Agent installs
* Adds L3 configuration option for specifying interface
  for external network bridge
Sysctl stuff belongs in node attrs/role attrs, not cookbook
Merge pull request #3 from jaypipes/working

Create the external network bridge in L3 agent